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1050 lines
39 KiB
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1050 lines
39 KiB
Plaintext
Please note that the "What is RCU?" LWN series is an excellent place
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to start learning about RCU:
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1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
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2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
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3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
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4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
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2010 Big API Table http://lwn.net/Articles/419086/
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5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/
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2014 Big API Table http://lwn.net/Articles/609973/
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What is RCU?
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RCU is a synchronization mechanism that was added to the Linux kernel
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during the 2.5 development effort that is optimized for read-mostly
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situations. Although RCU is actually quite simple once you understand it,
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getting there can sometimes be a challenge. Part of the problem is that
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most of the past descriptions of RCU have been written with the mistaken
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assumption that there is "one true way" to describe RCU. Instead,
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the experience has been that different people must take different paths
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to arrive at an understanding of RCU. This document provides several
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different paths, as follows:
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1. RCU OVERVIEW
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2. WHAT IS RCU'S CORE API?
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3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
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4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
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5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
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6. ANALOGY WITH READER-WRITER LOCKING
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7. FULL LIST OF RCU APIs
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8. ANSWERS TO QUICK QUIZZES
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People who prefer starting with a conceptual overview should focus on
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Section 1, though most readers will profit by reading this section at
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some point. People who prefer to start with an API that they can then
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experiment with should focus on Section 2. People who prefer to start
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with example uses should focus on Sections 3 and 4. People who need to
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understand the RCU implementation should focus on Section 5, then dive
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into the kernel source code. People who reason best by analogy should
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focus on Section 6. Section 7 serves as an index to the docbook API
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documentation, and Section 8 is the traditional answer key.
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So, start with the section that makes the most sense to you and your
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preferred method of learning. If you need to know everything about
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everything, feel free to read the whole thing -- but if you are really
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that type of person, you have perused the source code and will therefore
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never need this document anyway. ;-)
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1. RCU OVERVIEW
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The basic idea behind RCU is to split updates into "removal" and
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"reclamation" phases. The removal phase removes references to data items
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within a data structure (possibly by replacing them with references to
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new versions of these data items), and can run concurrently with readers.
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The reason that it is safe to run the removal phase concurrently with
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readers is the semantics of modern CPUs guarantee that readers will see
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either the old or the new version of the data structure rather than a
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partially updated reference. The reclamation phase does the work of reclaiming
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(e.g., freeing) the data items removed from the data structure during the
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removal phase. Because reclaiming data items can disrupt any readers
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concurrently referencing those data items, the reclamation phase must
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not start until readers no longer hold references to those data items.
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Splitting the update into removal and reclamation phases permits the
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updater to perform the removal phase immediately, and to defer the
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reclamation phase until all readers active during the removal phase have
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completed, either by blocking until they finish or by registering a
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callback that is invoked after they finish. Only readers that are active
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during the removal phase need be considered, because any reader starting
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after the removal phase will be unable to gain a reference to the removed
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data items, and therefore cannot be disrupted by the reclamation phase.
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So the typical RCU update sequence goes something like the following:
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a. Remove pointers to a data structure, so that subsequent
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readers cannot gain a reference to it.
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b. Wait for all previous readers to complete their RCU read-side
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critical sections.
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c. At this point, there cannot be any readers who hold references
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to the data structure, so it now may safely be reclaimed
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(e.g., kfree()d).
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Step (b) above is the key idea underlying RCU's deferred destruction.
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The ability to wait until all readers are done allows RCU readers to
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use much lighter-weight synchronization, in some cases, absolutely no
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synchronization at all. In contrast, in more conventional lock-based
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schemes, readers must use heavy-weight synchronization in order to
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prevent an updater from deleting the data structure out from under them.
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This is because lock-based updaters typically update data items in place,
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and must therefore exclude readers. In contrast, RCU-based updaters
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typically take advantage of the fact that writes to single aligned
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pointers are atomic on modern CPUs, allowing atomic insertion, removal,
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and replacement of data items in a linked structure without disrupting
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readers. Concurrent RCU readers can then continue accessing the old
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versions, and can dispense with the atomic operations, memory barriers,
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and communications cache misses that are so expensive on present-day
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SMP computer systems, even in absence of lock contention.
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In the three-step procedure shown above, the updater is performing both
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the removal and the reclamation step, but it is often helpful for an
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entirely different thread to do the reclamation, as is in fact the case
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in the Linux kernel's directory-entry cache (dcache). Even if the same
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thread performs both the update step (step (a) above) and the reclamation
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step (step (c) above), it is often helpful to think of them separately.
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For example, RCU readers and updaters need not communicate at all,
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but RCU provides implicit low-overhead communication between readers
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and reclaimers, namely, in step (b) above.
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So how the heck can a reclaimer tell when a reader is done, given
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that readers are not doing any sort of synchronization operations???
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Read on to learn about how RCU's API makes this easy.
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2. WHAT IS RCU'S CORE API?
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The core RCU API is quite small:
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a. rcu_read_lock()
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b. rcu_read_unlock()
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c. synchronize_rcu() / call_rcu()
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d. rcu_assign_pointer()
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e. rcu_dereference()
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There are many other members of the RCU API, but the rest can be
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expressed in terms of these five, though most implementations instead
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express synchronize_rcu() in terms of the call_rcu() callback API.
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The five core RCU APIs are described below, the other 18 will be enumerated
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later. See the kernel docbook documentation for more info, or look directly
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at the function header comments.
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rcu_read_lock()
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void rcu_read_lock(void);
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Used by a reader to inform the reclaimer that the reader is
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entering an RCU read-side critical section. It is illegal
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to block while in an RCU read-side critical section, though
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kernels built with CONFIG_PREEMPT_RCU can preempt RCU
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read-side critical sections. Any RCU-protected data structure
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accessed during an RCU read-side critical section is guaranteed to
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remain unreclaimed for the full duration of that critical section.
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Reference counts may be used in conjunction with RCU to maintain
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longer-term references to data structures.
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rcu_read_unlock()
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void rcu_read_unlock(void);
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Used by a reader to inform the reclaimer that the reader is
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exiting an RCU read-side critical section. Note that RCU
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read-side critical sections may be nested and/or overlapping.
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synchronize_rcu()
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void synchronize_rcu(void);
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Marks the end of updater code and the beginning of reclaimer
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code. It does this by blocking until all pre-existing RCU
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read-side critical sections on all CPUs have completed.
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Note that synchronize_rcu() will -not- necessarily wait for
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any subsequent RCU read-side critical sections to complete.
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For example, consider the following sequence of events:
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CPU 0 CPU 1 CPU 2
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----------------- ------------------------- ---------------
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1. rcu_read_lock()
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2. enters synchronize_rcu()
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3. rcu_read_lock()
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4. rcu_read_unlock()
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5. exits synchronize_rcu()
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6. rcu_read_unlock()
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To reiterate, synchronize_rcu() waits only for ongoing RCU
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read-side critical sections to complete, not necessarily for
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any that begin after synchronize_rcu() is invoked.
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Of course, synchronize_rcu() does not necessarily return
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-immediately- after the last pre-existing RCU read-side critical
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section completes. For one thing, there might well be scheduling
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delays. For another thing, many RCU implementations process
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requests in batches in order to improve efficiencies, which can
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further delay synchronize_rcu().
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Since synchronize_rcu() is the API that must figure out when
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readers are done, its implementation is key to RCU. For RCU
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to be useful in all but the most read-intensive situations,
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synchronize_rcu()'s overhead must also be quite small.
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The call_rcu() API is a callback form of synchronize_rcu(),
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and is described in more detail in a later section. Instead of
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blocking, it registers a function and argument which are invoked
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after all ongoing RCU read-side critical sections have completed.
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This callback variant is particularly useful in situations where
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it is illegal to block or where update-side performance is
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critically important.
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However, the call_rcu() API should not be used lightly, as use
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of the synchronize_rcu() API generally results in simpler code.
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In addition, the synchronize_rcu() API has the nice property
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of automatically limiting update rate should grace periods
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be delayed. This property results in system resilience in face
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of denial-of-service attacks. Code using call_rcu() should limit
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update rate in order to gain this same sort of resilience. See
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checklist.txt for some approaches to limiting the update rate.
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rcu_assign_pointer()
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typeof(p) rcu_assign_pointer(p, typeof(p) v);
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Yes, rcu_assign_pointer() -is- implemented as a macro, though it
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would be cool to be able to declare a function in this manner.
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(Compiler experts will no doubt disagree.)
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The updater uses this function to assign a new value to an
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RCU-protected pointer, in order to safely communicate the change
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in value from the updater to the reader. This function returns
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the new value, and also executes any memory-barrier instructions
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required for a given CPU architecture.
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Perhaps just as important, it serves to document (1) which
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pointers are protected by RCU and (2) the point at which a
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given structure becomes accessible to other CPUs. That said,
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rcu_assign_pointer() is most frequently used indirectly, via
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the _rcu list-manipulation primitives such as list_add_rcu().
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rcu_dereference()
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typeof(p) rcu_dereference(p);
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Like rcu_assign_pointer(), rcu_dereference() must be implemented
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as a macro.
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The reader uses rcu_dereference() to fetch an RCU-protected
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pointer, which returns a value that may then be safely
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dereferenced. Note that rcu_dereference() does not actually
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dereference the pointer, instead, it protects the pointer for
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later dereferencing. It also executes any needed memory-barrier
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instructions for a given CPU architecture. Currently, only Alpha
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needs memory barriers within rcu_dereference() -- on other CPUs,
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it compiles to nothing, not even a compiler directive.
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Common coding practice uses rcu_dereference() to copy an
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RCU-protected pointer to a local variable, then dereferences
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this local variable, for example as follows:
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p = rcu_dereference(head.next);
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return p->data;
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However, in this case, one could just as easily combine these
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into one statement:
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return rcu_dereference(head.next)->data;
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If you are going to be fetching multiple fields from the
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RCU-protected structure, using the local variable is of
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course preferred. Repeated rcu_dereference() calls look
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ugly, do not guarantee that the same pointer will be returned
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if an update happened while in the critical section, and incur
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unnecessary overhead on Alpha CPUs.
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Note that the value returned by rcu_dereference() is valid
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only within the enclosing RCU read-side critical section.
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For example, the following is -not- legal:
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rcu_read_lock();
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p = rcu_dereference(head.next);
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rcu_read_unlock();
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x = p->address; /* BUG!!! */
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rcu_read_lock();
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y = p->data; /* BUG!!! */
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rcu_read_unlock();
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Holding a reference from one RCU read-side critical section
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to another is just as illegal as holding a reference from
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one lock-based critical section to another! Similarly,
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using a reference outside of the critical section in which
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it was acquired is just as illegal as doing so with normal
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locking.
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As with rcu_assign_pointer(), an important function of
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rcu_dereference() is to document which pointers are protected by
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RCU, in particular, flagging a pointer that is subject to changing
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at any time, including immediately after the rcu_dereference().
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And, again like rcu_assign_pointer(), rcu_dereference() is
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typically used indirectly, via the _rcu list-manipulation
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primitives, such as list_for_each_entry_rcu().
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The following diagram shows how each API communicates among the
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reader, updater, and reclaimer.
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rcu_assign_pointer()
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+--------+
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+---------------------->| reader |---------+
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| +--------+ |
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| | |
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| | | Protect:
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| | | rcu_read_lock()
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| | | rcu_read_unlock()
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| rcu_dereference() | |
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+---------+ | |
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| updater |<---------------------+ |
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+---------+ V
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| +-----------+
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+----------------------------------->| reclaimer |
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+-----------+
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Defer:
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synchronize_rcu() & call_rcu()
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The RCU infrastructure observes the time sequence of rcu_read_lock(),
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rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
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order to determine when (1) synchronize_rcu() invocations may return
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to their callers and (2) call_rcu() callbacks may be invoked. Efficient
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implementations of the RCU infrastructure make heavy use of batching in
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order to amortize their overhead over many uses of the corresponding APIs.
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There are no fewer than three RCU mechanisms in the Linux kernel; the
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diagram above shows the first one, which is by far the most commonly used.
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The rcu_dereference() and rcu_assign_pointer() primitives are used for
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all three mechanisms, but different defer and protect primitives are
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used as follows:
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Defer Protect
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a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
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call_rcu() rcu_dereference()
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b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
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call_rcu_bh() rcu_dereference_bh()
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c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
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call_rcu_sched() preempt_disable() / preempt_enable()
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local_irq_save() / local_irq_restore()
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hardirq enter / hardirq exit
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NMI enter / NMI exit
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rcu_dereference_sched()
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These three mechanisms are used as follows:
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a. RCU applied to normal data structures.
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b. RCU applied to networking data structures that may be subjected
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to remote denial-of-service attacks.
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c. RCU applied to scheduler and interrupt/NMI-handler tasks.
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Again, most uses will be of (a). The (b) and (c) cases are important
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for specialized uses, but are relatively uncommon.
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3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
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This section shows a simple use of the core RCU API to protect a
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global pointer to a dynamically allocated structure. More-typical
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uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
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struct foo {
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int a;
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char b;
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long c;
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};
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DEFINE_SPINLOCK(foo_mutex);
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struct foo __rcu *gbl_foo;
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/*
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* Create a new struct foo that is the same as the one currently
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* pointed to by gbl_foo, except that field "a" is replaced
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* with "new_a". Points gbl_foo to the new structure, and
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* frees up the old structure after a grace period.
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*
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* Uses rcu_assign_pointer() to ensure that concurrent readers
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* see the initialized version of the new structure.
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*
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* Uses synchronize_rcu() to ensure that any readers that might
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* have references to the old structure complete before freeing
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* the old structure.
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*/
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void foo_update_a(int new_a)
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{
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struct foo *new_fp;
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struct foo *old_fp;
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new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
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spin_lock(&foo_mutex);
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old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
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*new_fp = *old_fp;
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new_fp->a = new_a;
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rcu_assign_pointer(gbl_foo, new_fp);
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spin_unlock(&foo_mutex);
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synchronize_rcu();
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kfree(old_fp);
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}
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/*
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* Return the value of field "a" of the current gbl_foo
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* structure. Use rcu_read_lock() and rcu_read_unlock()
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* to ensure that the structure does not get deleted out
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* from under us, and use rcu_dereference() to ensure that
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* we see the initialized version of the structure (important
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* for DEC Alpha and for people reading the code).
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*/
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int foo_get_a(void)
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{
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int retval;
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rcu_read_lock();
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retval = rcu_dereference(gbl_foo)->a;
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rcu_read_unlock();
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return retval;
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}
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So, to sum up:
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o Use rcu_read_lock() and rcu_read_unlock() to guard RCU
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read-side critical sections.
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o Within an RCU read-side critical section, use rcu_dereference()
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to dereference RCU-protected pointers.
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o Use some solid scheme (such as locks or semaphores) to
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keep concurrent updates from interfering with each other.
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o Use rcu_assign_pointer() to update an RCU-protected pointer.
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This primitive protects concurrent readers from the updater,
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-not- concurrent updates from each other! You therefore still
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need to use locking (or something similar) to keep concurrent
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rcu_assign_pointer() primitives from interfering with each other.
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o Use synchronize_rcu() -after- removing a data element from an
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RCU-protected data structure, but -before- reclaiming/freeing
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the data element, in order to wait for the completion of all
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RCU read-side critical sections that might be referencing that
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data item.
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See checklist.txt for additional rules to follow when using RCU.
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And again, more-typical uses of RCU may be found in listRCU.txt,
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arrayRCU.txt, and NMI-RCU.txt.
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4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
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In the example above, foo_update_a() blocks until a grace period elapses.
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This is quite simple, but in some cases one cannot afford to wait so
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long -- there might be other high-priority work to be done.
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In such cases, one uses call_rcu() rather than synchronize_rcu().
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The call_rcu() API is as follows:
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void call_rcu(struct rcu_head * head,
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void (*func)(struct rcu_head *head));
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This function invokes func(head) after a grace period has elapsed.
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This invocation might happen from either softirq or process context,
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so the function is not permitted to block. The foo struct needs to
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have an rcu_head structure added, perhaps as follows:
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struct foo {
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int a;
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char b;
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long c;
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struct rcu_head rcu;
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};
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The foo_update_a() function might then be written as follows:
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/*
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* Create a new struct foo that is the same as the one currently
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* pointed to by gbl_foo, except that field "a" is replaced
|
|
* with "new_a". Points gbl_foo to the new structure, and
|
|
* frees up the old structure after a grace period.
|
|
*
|
|
* Uses rcu_assign_pointer() to ensure that concurrent readers
|
|
* see the initialized version of the new structure.
|
|
*
|
|
* Uses call_rcu() to ensure that any readers that might have
|
|
* references to the old structure complete before freeing the
|
|
* old structure.
|
|
*/
|
|
void foo_update_a(int new_a)
|
|
{
|
|
struct foo *new_fp;
|
|
struct foo *old_fp;
|
|
|
|
new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
|
|
spin_lock(&foo_mutex);
|
|
old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
|
|
*new_fp = *old_fp;
|
|
new_fp->a = new_a;
|
|
rcu_assign_pointer(gbl_foo, new_fp);
|
|
spin_unlock(&foo_mutex);
|
|
call_rcu(&old_fp->rcu, foo_reclaim);
|
|
}
|
|
|
|
The foo_reclaim() function might appear as follows:
|
|
|
|
void foo_reclaim(struct rcu_head *rp)
|
|
{
|
|
struct foo *fp = container_of(rp, struct foo, rcu);
|
|
|
|
foo_cleanup(fp->a);
|
|
|
|
kfree(fp);
|
|
}
|
|
|
|
The container_of() primitive is a macro that, given a pointer into a
|
|
struct, the type of the struct, and the pointed-to field within the
|
|
struct, returns a pointer to the beginning of the struct.
|
|
|
|
The use of call_rcu() permits the caller of foo_update_a() to
|
|
immediately regain control, without needing to worry further about the
|
|
old version of the newly updated element. It also clearly shows the
|
|
RCU distinction between updater, namely foo_update_a(), and reclaimer,
|
|
namely foo_reclaim().
|
|
|
|
The summary of advice is the same as for the previous section, except
|
|
that we are now using call_rcu() rather than synchronize_rcu():
|
|
|
|
o Use call_rcu() -after- removing a data element from an
|
|
RCU-protected data structure in order to register a callback
|
|
function that will be invoked after the completion of all RCU
|
|
read-side critical sections that might be referencing that
|
|
data item.
|
|
|
|
If the callback for call_rcu() is not doing anything more than calling
|
|
kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
|
|
to avoid having to write your own callback:
|
|
|
|
kfree_rcu(old_fp, rcu);
|
|
|
|
Again, see checklist.txt for additional rules governing the use of RCU.
|
|
|
|
|
|
5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
|
|
|
|
One of the nice things about RCU is that it has extremely simple "toy"
|
|
implementations that are a good first step towards understanding the
|
|
production-quality implementations in the Linux kernel. This section
|
|
presents two such "toy" implementations of RCU, one that is implemented
|
|
in terms of familiar locking primitives, and another that more closely
|
|
resembles "classic" RCU. Both are way too simple for real-world use,
|
|
lacking both functionality and performance. However, they are useful
|
|
in getting a feel for how RCU works. See kernel/rcupdate.c for a
|
|
production-quality implementation, and see:
|
|
|
|
http://www.rdrop.com/users/paulmck/RCU
|
|
|
|
for papers describing the Linux kernel RCU implementation. The OLS'01
|
|
and OLS'02 papers are a good introduction, and the dissertation provides
|
|
more details on the current implementation as of early 2004.
|
|
|
|
|
|
5A. "TOY" IMPLEMENTATION #1: LOCKING
|
|
|
|
This section presents a "toy" RCU implementation that is based on
|
|
familiar locking primitives. Its overhead makes it a non-starter for
|
|
real-life use, as does its lack of scalability. It is also unsuitable
|
|
for realtime use, since it allows scheduling latency to "bleed" from
|
|
one read-side critical section to another. It also assumes recursive
|
|
reader-writer locks: If you try this with non-recursive locks, and
|
|
you allow nested rcu_read_lock() calls, you can deadlock.
|
|
|
|
However, it is probably the easiest implementation to relate to, so is
|
|
a good starting point.
|
|
|
|
It is extremely simple:
|
|
|
|
static DEFINE_RWLOCK(rcu_gp_mutex);
|
|
|
|
void rcu_read_lock(void)
|
|
{
|
|
read_lock(&rcu_gp_mutex);
|
|
}
|
|
|
|
void rcu_read_unlock(void)
|
|
{
|
|
read_unlock(&rcu_gp_mutex);
|
|
}
|
|
|
|
void synchronize_rcu(void)
|
|
{
|
|
write_lock(&rcu_gp_mutex);
|
|
write_unlock(&rcu_gp_mutex);
|
|
}
|
|
|
|
[You can ignore rcu_assign_pointer() and rcu_dereference() without missing
|
|
much. But here are simplified versions anyway. And whatever you do,
|
|
don't forget about them when submitting patches making use of RCU!]
|
|
|
|
#define rcu_assign_pointer(p, v) \
|
|
({ \
|
|
smp_store_release(&(p), (v)); \
|
|
})
|
|
|
|
#define rcu_dereference(p) \
|
|
({ \
|
|
typeof(p) _________p1 = p; \
|
|
smp_read_barrier_depends(); \
|
|
(_________p1); \
|
|
})
|
|
|
|
|
|
The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
|
|
and release a global reader-writer lock. The synchronize_rcu()
|
|
primitive write-acquires this same lock, then immediately releases
|
|
it. This means that once synchronize_rcu() exits, all RCU read-side
|
|
critical sections that were in progress before synchronize_rcu() was
|
|
called are guaranteed to have completed -- there is no way that
|
|
synchronize_rcu() would have been able to write-acquire the lock
|
|
otherwise.
|
|
|
|
It is possible to nest rcu_read_lock(), since reader-writer locks may
|
|
be recursively acquired. Note also that rcu_read_lock() is immune
|
|
from deadlock (an important property of RCU). The reason for this is
|
|
that the only thing that can block rcu_read_lock() is a synchronize_rcu().
|
|
But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
|
|
so there can be no deadlock cycle.
|
|
|
|
Quick Quiz #1: Why is this argument naive? How could a deadlock
|
|
occur when using this algorithm in a real-world Linux
|
|
kernel? How could this deadlock be avoided?
|
|
|
|
|
|
5B. "TOY" EXAMPLE #2: CLASSIC RCU
|
|
|
|
This section presents a "toy" RCU implementation that is based on
|
|
"classic RCU". It is also short on performance (but only for updates) and
|
|
on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
|
|
kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
|
|
are the same as those shown in the preceding section, so they are omitted.
|
|
|
|
void rcu_read_lock(void) { }
|
|
|
|
void rcu_read_unlock(void) { }
|
|
|
|
void synchronize_rcu(void)
|
|
{
|
|
int cpu;
|
|
|
|
for_each_possible_cpu(cpu)
|
|
run_on(cpu);
|
|
}
|
|
|
|
Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
|
|
This is the great strength of classic RCU in a non-preemptive kernel:
|
|
read-side overhead is precisely zero, at least on non-Alpha CPUs.
|
|
And there is absolutely no way that rcu_read_lock() can possibly
|
|
participate in a deadlock cycle!
|
|
|
|
The implementation of synchronize_rcu() simply schedules itself on each
|
|
CPU in turn. The run_on() primitive can be implemented straightforwardly
|
|
in terms of the sched_setaffinity() primitive. Of course, a somewhat less
|
|
"toy" implementation would restore the affinity upon completion rather
|
|
than just leaving all tasks running on the last CPU, but when I said
|
|
"toy", I meant -toy-!
|
|
|
|
So how the heck is this supposed to work???
|
|
|
|
Remember that it is illegal to block while in an RCU read-side critical
|
|
section. Therefore, if a given CPU executes a context switch, we know
|
|
that it must have completed all preceding RCU read-side critical sections.
|
|
Once -all- CPUs have executed a context switch, then -all- preceding
|
|
RCU read-side critical sections will have completed.
|
|
|
|
So, suppose that we remove a data item from its structure and then invoke
|
|
synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
|
|
that there are no RCU read-side critical sections holding a reference
|
|
to that data item, so we can safely reclaim it.
|
|
|
|
Quick Quiz #2: Give an example where Classic RCU's read-side
|
|
overhead is -negative-.
|
|
|
|
Quick Quiz #3: If it is illegal to block in an RCU read-side
|
|
critical section, what the heck do you do in
|
|
PREEMPT_RT, where normal spinlocks can block???
|
|
|
|
|
|
6. ANALOGY WITH READER-WRITER LOCKING
|
|
|
|
Although RCU can be used in many different ways, a very common use of
|
|
RCU is analogous to reader-writer locking. The following unified
|
|
diff shows how closely related RCU and reader-writer locking can be.
|
|
|
|
@@ -5,5 +5,5 @@ struct el {
|
|
int data;
|
|
/* Other data fields */
|
|
};
|
|
-rwlock_t listmutex;
|
|
+spinlock_t listmutex;
|
|
struct el head;
|
|
|
|
@@ -13,15 +14,15 @@
|
|
struct list_head *lp;
|
|
struct el *p;
|
|
|
|
- read_lock(&listmutex);
|
|
- list_for_each_entry(p, head, lp) {
|
|
+ rcu_read_lock();
|
|
+ list_for_each_entry_rcu(p, head, lp) {
|
|
if (p->key == key) {
|
|
*result = p->data;
|
|
- read_unlock(&listmutex);
|
|
+ rcu_read_unlock();
|
|
return 1;
|
|
}
|
|
}
|
|
- read_unlock(&listmutex);
|
|
+ rcu_read_unlock();
|
|
return 0;
|
|
}
|
|
|
|
@@ -29,15 +30,16 @@
|
|
{
|
|
struct el *p;
|
|
|
|
- write_lock(&listmutex);
|
|
+ spin_lock(&listmutex);
|
|
list_for_each_entry(p, head, lp) {
|
|
if (p->key == key) {
|
|
- list_del(&p->list);
|
|
- write_unlock(&listmutex);
|
|
+ list_del_rcu(&p->list);
|
|
+ spin_unlock(&listmutex);
|
|
+ synchronize_rcu();
|
|
kfree(p);
|
|
return 1;
|
|
}
|
|
}
|
|
- write_unlock(&listmutex);
|
|
+ spin_unlock(&listmutex);
|
|
return 0;
|
|
}
|
|
|
|
Or, for those who prefer a side-by-side listing:
|
|
|
|
1 struct el { 1 struct el {
|
|
2 struct list_head list; 2 struct list_head list;
|
|
3 long key; 3 long key;
|
|
4 spinlock_t mutex; 4 spinlock_t mutex;
|
|
5 int data; 5 int data;
|
|
6 /* Other data fields */ 6 /* Other data fields */
|
|
7 }; 7 };
|
|
8 rwlock_t listmutex; 8 spinlock_t listmutex;
|
|
9 struct el head; 9 struct el head;
|
|
|
|
1 int search(long key, int *result) 1 int search(long key, int *result)
|
|
2 { 2 {
|
|
3 struct list_head *lp; 3 struct list_head *lp;
|
|
4 struct el *p; 4 struct el *p;
|
|
5 5
|
|
6 read_lock(&listmutex); 6 rcu_read_lock();
|
|
7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
|
|
8 if (p->key == key) { 8 if (p->key == key) {
|
|
9 *result = p->data; 9 *result = p->data;
|
|
10 read_unlock(&listmutex); 10 rcu_read_unlock();
|
|
11 return 1; 11 return 1;
|
|
12 } 12 }
|
|
13 } 13 }
|
|
14 read_unlock(&listmutex); 14 rcu_read_unlock();
|
|
15 return 0; 15 return 0;
|
|
16 } 16 }
|
|
|
|
1 int delete(long key) 1 int delete(long key)
|
|
2 { 2 {
|
|
3 struct el *p; 3 struct el *p;
|
|
4 4
|
|
5 write_lock(&listmutex); 5 spin_lock(&listmutex);
|
|
6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
|
|
7 if (p->key == key) { 7 if (p->key == key) {
|
|
8 list_del(&p->list); 8 list_del_rcu(&p->list);
|
|
9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
|
|
10 synchronize_rcu();
|
|
10 kfree(p); 11 kfree(p);
|
|
11 return 1; 12 return 1;
|
|
12 } 13 }
|
|
13 } 14 }
|
|
14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
|
|
15 return 0; 16 return 0;
|
|
16 } 17 }
|
|
|
|
Either way, the differences are quite small. Read-side locking moves
|
|
to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
|
|
a reader-writer lock to a simple spinlock, and a synchronize_rcu()
|
|
precedes the kfree().
|
|
|
|
However, there is one potential catch: the read-side and update-side
|
|
critical sections can now run concurrently. In many cases, this will
|
|
not be a problem, but it is necessary to check carefully regardless.
|
|
For example, if multiple independent list updates must be seen as
|
|
a single atomic update, converting to RCU will require special care.
|
|
|
|
Also, the presence of synchronize_rcu() means that the RCU version of
|
|
delete() can now block. If this is a problem, there is a callback-based
|
|
mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
|
|
be used in place of synchronize_rcu().
|
|
|
|
|
|
7. FULL LIST OF RCU APIs
|
|
|
|
The RCU APIs are documented in docbook-format header comments in the
|
|
Linux-kernel source code, but it helps to have a full list of the
|
|
APIs, since there does not appear to be a way to categorize them
|
|
in docbook. Here is the list, by category.
|
|
|
|
RCU list traversal:
|
|
|
|
list_entry_rcu
|
|
list_first_entry_rcu
|
|
list_next_rcu
|
|
list_for_each_entry_rcu
|
|
list_for_each_entry_continue_rcu
|
|
hlist_first_rcu
|
|
hlist_next_rcu
|
|
hlist_pprev_rcu
|
|
hlist_for_each_entry_rcu
|
|
hlist_for_each_entry_rcu_bh
|
|
hlist_for_each_entry_continue_rcu
|
|
hlist_for_each_entry_continue_rcu_bh
|
|
hlist_nulls_first_rcu
|
|
hlist_nulls_for_each_entry_rcu
|
|
hlist_bl_first_rcu
|
|
hlist_bl_for_each_entry_rcu
|
|
|
|
RCU pointer/list update:
|
|
|
|
rcu_assign_pointer
|
|
list_add_rcu
|
|
list_add_tail_rcu
|
|
list_del_rcu
|
|
list_replace_rcu
|
|
hlist_add_behind_rcu
|
|
hlist_add_before_rcu
|
|
hlist_add_head_rcu
|
|
hlist_del_rcu
|
|
hlist_del_init_rcu
|
|
hlist_replace_rcu
|
|
list_splice_init_rcu()
|
|
hlist_nulls_del_init_rcu
|
|
hlist_nulls_del_rcu
|
|
hlist_nulls_add_head_rcu
|
|
hlist_bl_add_head_rcu
|
|
hlist_bl_del_init_rcu
|
|
hlist_bl_del_rcu
|
|
hlist_bl_set_first_rcu
|
|
|
|
RCU: Critical sections Grace period Barrier
|
|
|
|
rcu_read_lock synchronize_net rcu_barrier
|
|
rcu_read_unlock synchronize_rcu
|
|
rcu_dereference synchronize_rcu_expedited
|
|
rcu_read_lock_held call_rcu
|
|
rcu_dereference_check kfree_rcu
|
|
rcu_dereference_protected
|
|
|
|
bh: Critical sections Grace period Barrier
|
|
|
|
rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
|
|
rcu_read_unlock_bh synchronize_rcu_bh
|
|
rcu_dereference_bh synchronize_rcu_bh_expedited
|
|
rcu_dereference_bh_check
|
|
rcu_dereference_bh_protected
|
|
rcu_read_lock_bh_held
|
|
|
|
sched: Critical sections Grace period Barrier
|
|
|
|
rcu_read_lock_sched synchronize_sched rcu_barrier_sched
|
|
rcu_read_unlock_sched call_rcu_sched
|
|
[preempt_disable] synchronize_sched_expedited
|
|
[and friends]
|
|
rcu_read_lock_sched_notrace
|
|
rcu_read_unlock_sched_notrace
|
|
rcu_dereference_sched
|
|
rcu_dereference_sched_check
|
|
rcu_dereference_sched_protected
|
|
rcu_read_lock_sched_held
|
|
|
|
|
|
SRCU: Critical sections Grace period Barrier
|
|
|
|
srcu_read_lock synchronize_srcu srcu_barrier
|
|
srcu_read_unlock call_srcu
|
|
srcu_dereference synchronize_srcu_expedited
|
|
srcu_dereference_check
|
|
srcu_read_lock_held
|
|
|
|
SRCU: Initialization/cleanup
|
|
DEFINE_SRCU
|
|
DEFINE_STATIC_SRCU
|
|
init_srcu_struct
|
|
cleanup_srcu_struct
|
|
|
|
All: lockdep-checked RCU-protected pointer access
|
|
|
|
rcu_access_pointer
|
|
rcu_dereference_raw
|
|
RCU_LOCKDEP_WARN
|
|
rcu_sleep_check
|
|
RCU_NONIDLE
|
|
|
|
See the comment headers in the source code (or the docbook generated
|
|
from them) for more information.
|
|
|
|
However, given that there are no fewer than four families of RCU APIs
|
|
in the Linux kernel, how do you choose which one to use? The following
|
|
list can be helpful:
|
|
|
|
a. Will readers need to block? If so, you need SRCU.
|
|
|
|
b. What about the -rt patchset? If readers would need to block
|
|
in an non-rt kernel, you need SRCU. If readers would block
|
|
in a -rt kernel, but not in a non-rt kernel, SRCU is not
|
|
necessary. (The -rt patchset turns spinlocks into sleeplocks,
|
|
hence this distinction.)
|
|
|
|
c. Do you need to treat NMI handlers, hardirq handlers,
|
|
and code segments with preemption disabled (whether
|
|
via preempt_disable(), local_irq_save(), local_bh_disable(),
|
|
or some other mechanism) as if they were explicit RCU readers?
|
|
If so, RCU-sched is the only choice that will work for you.
|
|
|
|
d. Do you need RCU grace periods to complete even in the face
|
|
of softirq monopolization of one or more of the CPUs? For
|
|
example, is your code subject to network-based denial-of-service
|
|
attacks? If so, you need RCU-bh.
|
|
|
|
e. Is your workload too update-intensive for normal use of
|
|
RCU, but inappropriate for other synchronization mechanisms?
|
|
If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
|
|
named SLAB_DESTROY_BY_RCU). But please be careful!
|
|
|
|
f. Do you need read-side critical sections that are respected
|
|
even though they are in the middle of the idle loop, during
|
|
user-mode execution, or on an offlined CPU? If so, SRCU is the
|
|
only choice that will work for you.
|
|
|
|
g. Otherwise, use RCU.
|
|
|
|
Of course, this all assumes that you have determined that RCU is in fact
|
|
the right tool for your job.
|
|
|
|
|
|
8. ANSWERS TO QUICK QUIZZES
|
|
|
|
Quick Quiz #1: Why is this argument naive? How could a deadlock
|
|
occur when using this algorithm in a real-world Linux
|
|
kernel? [Referring to the lock-based "toy" RCU
|
|
algorithm.]
|
|
|
|
Answer: Consider the following sequence of events:
|
|
|
|
1. CPU 0 acquires some unrelated lock, call it
|
|
"problematic_lock", disabling irq via
|
|
spin_lock_irqsave().
|
|
|
|
2. CPU 1 enters synchronize_rcu(), write-acquiring
|
|
rcu_gp_mutex.
|
|
|
|
3. CPU 0 enters rcu_read_lock(), but must wait
|
|
because CPU 1 holds rcu_gp_mutex.
|
|
|
|
4. CPU 1 is interrupted, and the irq handler
|
|
attempts to acquire problematic_lock.
|
|
|
|
The system is now deadlocked.
|
|
|
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One way to avoid this deadlock is to use an approach like
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that of CONFIG_PREEMPT_RT, where all normal spinlocks
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become blocking locks, and all irq handlers execute in
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the context of special tasks. In this case, in step 4
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above, the irq handler would block, allowing CPU 1 to
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release rcu_gp_mutex, avoiding the deadlock.
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Even in the absence of deadlock, this RCU implementation
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allows latency to "bleed" from readers to other
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readers through synchronize_rcu(). To see this,
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consider task A in an RCU read-side critical section
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(thus read-holding rcu_gp_mutex), task B blocked
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attempting to write-acquire rcu_gp_mutex, and
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task C blocked in rcu_read_lock() attempting to
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read_acquire rcu_gp_mutex. Task A's RCU read-side
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latency is holding up task C, albeit indirectly via
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task B.
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Realtime RCU implementations therefore use a counter-based
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approach where tasks in RCU read-side critical sections
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cannot be blocked by tasks executing synchronize_rcu().
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Quick Quiz #2: Give an example where Classic RCU's read-side
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overhead is -negative-.
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Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT
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kernel where a routing table is used by process-context
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code, but can be updated by irq-context code (for example,
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by an "ICMP REDIRECT" packet). The usual way of handling
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this would be to have the process-context code disable
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interrupts while searching the routing table. Use of
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RCU allows such interrupt-disabling to be dispensed with.
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Thus, without RCU, you pay the cost of disabling interrupts,
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and with RCU you don't.
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One can argue that the overhead of RCU in this
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case is negative with respect to the single-CPU
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interrupt-disabling approach. Others might argue that
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the overhead of RCU is merely zero, and that replacing
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the positive overhead of the interrupt-disabling scheme
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with the zero-overhead RCU scheme does not constitute
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negative overhead.
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In real life, of course, things are more complex. But
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even the theoretical possibility of negative overhead for
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a synchronization primitive is a bit unexpected. ;-)
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Quick Quiz #3: If it is illegal to block in an RCU read-side
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critical section, what the heck do you do in
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PREEMPT_RT, where normal spinlocks can block???
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Answer: Just as PREEMPT_RT permits preemption of spinlock
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critical sections, it permits preemption of RCU
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read-side critical sections. It also permits
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spinlocks blocking while in RCU read-side critical
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sections.
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Why the apparent inconsistency? Because it is it
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possible to use priority boosting to keep the RCU
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grace periods short if need be (for example, if running
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short of memory). In contrast, if blocking waiting
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for (say) network reception, there is no way to know
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what should be boosted. Especially given that the
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process we need to boost might well be a human being
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who just went out for a pizza or something. And although
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a computer-operated cattle prod might arouse serious
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interest, it might also provoke serious objections.
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Besides, how does the computer know what pizza parlor
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the human being went to???
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ACKNOWLEDGEMENTS
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My thanks to the people who helped make this human-readable, including
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Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
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For more information, see http://www.rdrop.com/users/paulmck/RCU.
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