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300 lines
10 KiB
ReStructuredText
300 lines
10 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. _kernel_hacking_locktypes:
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==========================
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Lock types and their rules
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==========================
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Introduction
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============
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The kernel provides a variety of locking primitives which can be divided
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into two categories:
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- Sleeping locks
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- Spinning locks
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This document conceptually describes these lock types and provides rules
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for their nesting, including the rules for use under PREEMPT_RT.
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Lock categories
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===============
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Sleeping locks
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--------------
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Sleeping locks can only be acquired in preemptible task context.
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Although implementations allow try_lock() from other contexts, it is
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necessary to carefully evaluate the safety of unlock() as well as of
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try_lock(). Furthermore, it is also necessary to evaluate the debugging
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versions of these primitives. In short, don't acquire sleeping locks from
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other contexts unless there is no other option.
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Sleeping lock types:
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- mutex
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- rt_mutex
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- semaphore
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- rw_semaphore
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- ww_mutex
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- percpu_rw_semaphore
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On PREEMPT_RT kernels, these lock types are converted to sleeping locks:
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- spinlock_t
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- rwlock_t
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Spinning locks
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--------------
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- raw_spinlock_t
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- bit spinlocks
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On non-PREEMPT_RT kernels, these lock types are also spinning locks:
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- spinlock_t
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- rwlock_t
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Spinning locks implicitly disable preemption and the lock / unlock functions
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can have suffixes which apply further protections:
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=================== ====================================================
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_bh() Disable / enable bottom halves (soft interrupts)
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_irq() Disable / enable interrupts
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_irqsave/restore() Save and disable / restore interrupt disabled state
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=================== ====================================================
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rtmutex
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=======
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RT-mutexes are mutexes with support for priority inheritance (PI).
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PI has limitations on non PREEMPT_RT enabled kernels due to preemption and
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interrupt disabled sections.
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PI clearly cannot preempt preemption-disabled or interrupt-disabled
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regions of code, even on PREEMPT_RT kernels. Instead, PREEMPT_RT kernels
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execute most such regions of code in preemptible task context, especially
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interrupt handlers and soft interrupts. This conversion allows spinlock_t
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and rwlock_t to be implemented via RT-mutexes.
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raw_spinlock_t and spinlock_t
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=============================
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raw_spinlock_t
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--------------
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raw_spinlock_t is a strict spinning lock implementation regardless of the
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kernel configuration including PREEMPT_RT enabled kernels.
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raw_spinlock_t is a strict spinning lock implementation in all kernels,
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including PREEMPT_RT kernels. Use raw_spinlock_t only in real critical
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core code, low level interrupt handling and places where disabling
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preemption or interrupts is required, for example, to safely access
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hardware state. raw_spinlock_t can sometimes also be used when the
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critical section is tiny, thus avoiding RT-mutex overhead.
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spinlock_t
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----------
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The semantics of spinlock_t change with the state of CONFIG_PREEMPT_RT.
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On a non PREEMPT_RT enabled kernel spinlock_t is mapped to raw_spinlock_t
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and has exactly the same semantics.
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spinlock_t and PREEMPT_RT
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-------------------------
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On a PREEMPT_RT enabled kernel spinlock_t is mapped to a separate
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implementation based on rt_mutex which changes the semantics:
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- Preemption is not disabled
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- The hard interrupt related suffixes for spin_lock / spin_unlock
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operations (_irq, _irqsave / _irqrestore) do not affect the CPUs
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interrupt disabled state
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- The soft interrupt related suffix (_bh()) still disables softirq
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handlers.
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Non-PREEMPT_RT kernels disable preemption to get this effect.
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PREEMPT_RT kernels use a per-CPU lock for serialization which keeps
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preemption disabled. The lock disables softirq handlers and also
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prevents reentrancy due to task preemption.
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PREEMPT_RT kernels preserve all other spinlock_t semantics:
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- Tasks holding a spinlock_t do not migrate. Non-PREEMPT_RT kernels
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avoid migration by disabling preemption. PREEMPT_RT kernels instead
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disable migration, which ensures that pointers to per-CPU variables
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remain valid even if the task is preempted.
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- Task state is preserved across spinlock acquisition, ensuring that the
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task-state rules apply to all kernel configurations. Non-PREEMPT_RT
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kernels leave task state untouched. However, PREEMPT_RT must change
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task state if the task blocks during acquisition. Therefore, it saves
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the current task state before blocking and the corresponding lock wakeup
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restores it.
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Other types of wakeups would normally unconditionally set the task state
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to RUNNING, but that does not work here because the task must remain
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blocked until the lock becomes available. Therefore, when a non-lock
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wakeup attempts to awaken a task blocked waiting for a spinlock, it
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instead sets the saved state to RUNNING. Then, when the lock
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acquisition completes, the lock wakeup sets the task state to the saved
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state, in this case setting it to RUNNING.
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rwlock_t
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========
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rwlock_t is a multiple readers and single writer lock mechanism.
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Non-PREEMPT_RT kernels implement rwlock_t as a spinning lock and the
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suffix rules of spinlock_t apply accordingly. The implementation is fair,
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thus preventing writer starvation.
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rwlock_t and PREEMPT_RT
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-----------------------
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PREEMPT_RT kernels map rwlock_t to a separate rt_mutex-based
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implementation, thus changing semantics:
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- All the spinlock_t changes also apply to rwlock_t.
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- Because an rwlock_t writer cannot grant its priority to multiple
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readers, a preempted low-priority reader will continue holding its lock,
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thus starving even high-priority writers. In contrast, because readers
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can grant their priority to a writer, a preempted low-priority writer
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will have its priority boosted until it releases the lock, thus
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preventing that writer from starving readers.
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PREEMPT_RT caveats
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==================
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spinlock_t and rwlock_t
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-----------------------
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These changes in spinlock_t and rwlock_t semantics on PREEMPT_RT kernels
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have a few implications. For example, on a non-PREEMPT_RT kernel the
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following code sequence works as expected::
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local_irq_disable();
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spin_lock(&lock);
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and is fully equivalent to::
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spin_lock_irq(&lock);
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Same applies to rwlock_t and the _irqsave() suffix variants.
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On PREEMPT_RT kernel this code sequence breaks because RT-mutex requires a
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fully preemptible context. Instead, use spin_lock_irq() or
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spin_lock_irqsave() and their unlock counterparts. In cases where the
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interrupt disabling and locking must remain separate, PREEMPT_RT offers a
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local_lock mechanism. Acquiring the local_lock pins the task to a CPU,
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allowing things like per-CPU irq-disabled locks to be acquired. However,
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this approach should be used only where absolutely necessary.
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raw_spinlock_t
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--------------
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Acquiring a raw_spinlock_t disables preemption and possibly also
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interrupts, so the critical section must avoid acquiring a regular
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spinlock_t or rwlock_t, for example, the critical section must avoid
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allocating memory. Thus, on a non-PREEMPT_RT kernel the following code
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works perfectly::
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raw_spin_lock(&lock);
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p = kmalloc(sizeof(*p), GFP_ATOMIC);
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But this code fails on PREEMPT_RT kernels because the memory allocator is
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fully preemptible and therefore cannot be invoked from truly atomic
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contexts. However, it is perfectly fine to invoke the memory allocator
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while holding normal non-raw spinlocks because they do not disable
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preemption on PREEMPT_RT kernels::
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spin_lock(&lock);
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p = kmalloc(sizeof(*p), GFP_ATOMIC);
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bit spinlocks
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-------------
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Bit spinlocks are problematic for PREEMPT_RT as they cannot be easily
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substituted by an RT-mutex based implementation for obvious reasons.
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The semantics of bit spinlocks are preserved on PREEMPT_RT kernels and the
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caveats vs. raw_spinlock_t apply.
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Some bit spinlocks are substituted by regular spinlock_t for PREEMPT_RT but
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this requires conditional (#ifdef'ed) code changes at the usage site while
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the spinlock_t substitution is simply done by the compiler and the
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conditionals are restricted to header files and core implementation of the
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locking primitives and the usage sites do not require any changes.
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Lock type nesting rules
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=======================
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The most basic rules are:
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- Lock types of the same lock category (sleeping, spinning) can nest
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arbitrarily as long as they respect the general lock ordering rules to
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prevent deadlocks.
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- Sleeping lock types cannot nest inside spinning lock types.
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- Spinning lock types can nest inside sleeping lock types.
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These rules apply in general independent of CONFIG_PREEMPT_RT.
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As PREEMPT_RT changes the lock category of spinlock_t and rwlock_t from
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spinning to sleeping this has obviously restrictions how they can nest with
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raw_spinlock_t.
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This results in the following nest ordering:
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1) Sleeping locks
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2) spinlock_t and rwlock_t
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3) raw_spinlock_t and bit spinlocks
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Lockdep is aware of these constraints to ensure that they are respected.
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Owner semantics
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===============
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Most lock types in the Linux kernel have strict owner semantics, i.e. the
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context (task) which acquires a lock has to release it.
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There are two exceptions:
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- semaphores
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- rwsems
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semaphores have no owner semantics for historical reason, and as such
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trylock and release operations can be called from any context. They are
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often used for both serialization and waiting purposes. That's generally
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discouraged and should be replaced by separate serialization and wait
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mechanisms, such as mutexes and completions.
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rwsems have grown interfaces which allow non owner release for special
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purposes. This usage is problematic on PREEMPT_RT because PREEMPT_RT
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substitutes all locking primitives except semaphores with RT-mutex based
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implementations to provide priority inheritance for all lock types except
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the truly spinning ones. Priority inheritance on ownerless locks is
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obviously impossible.
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For now the rwsem non-owner release excludes code which utilizes it from
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being used on PREEMPT_RT enabled kernels. In same cases this can be
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mitigated by disabling portions of the code, in other cases the complete
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functionality has to be disabled until a workable solution has been found.
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