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Revert docs from "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()" 

This reverts docs from commit d6b9cd7dc8e041ee83cb1362fce59a3cdb1f2709.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Added Joel's SoB per Stephen Rothwell feedback. ]
[ paulmck: Joel approved via private email. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Joel Fernandes (Google) 2019-08-01 17:39:16 -04:00 committed by Paul E. McKenney
parent 54ecb8f702
commit 97df75cde5
1 changed files with 0 additions and 71 deletions
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Documentation/RCU/Design/Requirements/Requirements.html
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@ -2129,8 +2129,6 @@ Some of the relevant points of interest are as follows:
<li> <a href="#Hotplug CPU">Hotplug CPU</a>.
<li> <a href="#Scheduler and RCU">Scheduler and RCU</a>.
<li> <a href="#Tracing and RCU">Tracing and RCU</a>.
<li> <a href="#Accesses to User Memory and RCU">
Accesses to User Memory and RCU</a>.
<li> <a href="#Energy Efficiency">Energy Efficiency</a>.
<li> <a href="#Scheduling-Clock Interrupts and RCU">
Scheduling-Clock Interrupts and RCU</a>.
@ -2523,75 +2521,6 @@ cannot be used.
The tracing folks both located the requirement and provided the
needed fix, so this surprise requirement was relatively painless.
<h3><a name="Accesses to User Memory and RCU">
Accesses to User Memory and RCU</a></h3>
<p>
The kernel needs to access user-space memory, for example, to access
data referenced by system-call parameters.
The <tt>get_user()</tt> macro does this job.
<p>
However, user-space memory might well be paged out, which means
that <tt>get_user()</tt> might well page-fault and thus block while
waiting for the resulting I/O to complete.
It would be a very bad thing for the compiler to reorder
a <tt>get_user()</tt> invocation into an RCU read-side critical
section.
For example, suppose that the source code looked like this:
<blockquote>
<pre>
1 rcu_read_lock();
2 p = rcu_dereference(gp);
3 v = p-&gt;value;
4 rcu_read_unlock();
5 get_user(user_v, user_p);
6 do_something_with(v, user_v);
</pre>
</blockquote>
<p>
The compiler must not be permitted to transform this source code into
the following:
<blockquote>
<pre>
1 rcu_read_lock();
2 p = rcu_dereference(gp);
3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!
4 v = p-&gt;value;
5 rcu_read_unlock();
6 do_something_with(v, user_v);
</pre>
</blockquote>
<p>
If the compiler did make this transformation in a
<tt>CONFIG_PREEMPT=n</tt> kernel build, and if <tt>get_user()</tt> did
page fault, the result would be a quiescent state in the middle
of an RCU read-side critical section.
This misplaced quiescent state could result in line&nbsp;4 being
a use-after-free access, which could be bad for your kernel's
actuarial statistics.
Similar examples can be constructed with the call to <tt>get_user()</tt>
preceding the <tt>rcu_read_lock()</tt>.
<p>
Unfortunately, <tt>get_user()</tt> doesn't have any particular
ordering properties, and in some architectures the underlying <tt>asm</tt>
isn't even marked <tt>volatile</tt>.
And even if it was marked <tt>volatile</tt>, the above access to
<tt>p-&gt;value</tt> is not volatile, so the compiler would not have any
reason to keep those two accesses in order.
<p>
Therefore, the Linux-kernel definitions of <tt>rcu_read_lock()</tt>
and <tt>rcu_read_unlock()</tt> must act as compiler barriers,
at least for outermost instances of <tt>rcu_read_lock()</tt> and
<tt>rcu_read_unlock()</tt> within a nested set of RCU read-side critical
sections.
<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
<p>