linux-kernel - [PATCH 3.16 089/305] sched/preempt: Fix preempt

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Date:	Sat, 13 Aug 2016 18:42:51 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Vikram Mulukutla" <markivx@...eaurora.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Ingo Molnar" <mingo@...nel.org>
Subject: [PATCH 3.16 089/305] sched/preempt: Fix preempt_count manipulations

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@...radead.org>

commit 2e636d5e66c35dfcbaf617aa8fa963f6847478fe upstream.

Vikram reported that his ARM64 compiler managed to 'optimize' away the
preempt_count manipulations in code like:

	preempt_enable_no_resched();
	put_user();
	preempt_disable();

Irrespective of that fact that that is horrible code that should be
fixed for many reasons, it does highlight a deficiency in the generic
preempt_count manipulators. As it is never right to combine/elide
preempt_count manipulations like this.

Therefore sprinkle some volatile in the two generic accessors to
ensure the compiler is aware of the fact that the preempt_count is
observed outside of the regular program-order view and thus cannot be
optimized away like this.

x86; the only arch not using the generic code is not affected as we
do all this in asm in order to use the segment base per-cpu stuff.

Reported-by: Vikram Mulukutla <markivx@...eaurora.org>
Tested-by: Vikram Mulukutla <markivx@...eaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: a787870924db ("sched, arch: Create asm/preempt.h")
Link: http://lkml.kernel.org/r/20160516131751.GH3205@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[bwh: Backported to 3.16: use ACCESS_ONCE() instead of READ_ONCE()]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/asm-generic/preempt.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -7,10 +7,10 @@

 static __always_inline int preempt_count(void)
 {
-	return current_thread_info()->preempt_count;
+	return ACCESS_ONCE(current_thread_info()->preempt_count);
 }

-static __always_inline int *preempt_count_ptr(void)
+static __always_inline volatile int *preempt_count_ptr(void)
 {
 	return &current_thread_info()->preempt_count;
 }