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From: paulmck@kernel.org
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
	fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
	"Paul E . McKenney" <paulmck@linux.ibm.com>
Subject: [PATCH tip/core/rcu 1/9] Revert docs from "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()"
Date: Wed,  2 Oct 2019 18:28:07 -0700
Message-ID: <20191003012815.12639-1-paulmck@kernel.org> (raw)
In-Reply-To: <20191003012741.GA12456@paulmck-ThinkPad-P72>

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

This reverts docs from commit d6b9cd7dc8e041ee83cb1362fce59a3cdb1f2709.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
 .../RCU/Design/Requirements/Requirements.html      | 71 ----------------------
 1 file changed, 71 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 467251f..bdbc84f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -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>
-- 
2.9.5


  reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-03  1:27 [PATCH tip/core/rcu 0/9] Documentation updates for v5.5 Paul E. McKenney
2019-10-03  1:28 ` paulmck [this message]
2019-10-03  1:28 ` [PATCH tip/core/rcu 2/9] Revert docs from "treewide: Rename rcu_dereference_raw_notrace() to _check()" paulmck
2019-10-03  1:28 ` [PATCH tip/core/rcu 4/9] docs: rcu: Correct links referring to titles paulmck
2019-10-03  1:28 ` [PATCH tip/core/rcu 5/9] docs: rcu: Increase toctree to 3 paulmck
2019-10-03  1:28 ` [PATCH tip/core/rcu 6/9] Restore docs "treewide: Rename rcu_dereference_raw_notrace() to _check()" paulmck
2019-10-03  1:28 ` [PATCH tip/core/rcu 7/9] Restore docs "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()" paulmck
2019-10-03  1:28 ` [PATCH tip/core/rcu 8/9] doc: Update list_for_each_entry_rcu() documentation paulmck
2019-10-03  1:28 ` [PATCH tip/core/rcu 9/9] Documentation: Rename rcu_node_context_switch() to rcu_note_context_switch() paulmck

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