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sched/preempt: Fix preempt_count manipulations

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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@codeaurora.org>
Tested-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.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@kernel.org>
This commit is contained in:
Peter Zijlstra 2016-05-16 15:01:11 +02:00 committed by Ingo Molnar
parent bc231d9ede
commit 2e636d5e66
1 changed files with 2 additions and 2 deletions
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4
include/asm-generic/preempt.h
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@ -7,10 +7,10 @@
static __always_inline int preempt_count(void)
{
return current_thread_info()->preempt_count;
return READ_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;
}