Messages in this thread Patch in this message | | | Date | Tue, 17 May 2016 07:21:40 -0700 | From | tip-bot for Peter Zijlstra <> | Subject | [tip:sched/urgent] sched/preempt: Fix preempt_count manipulations |
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Commit-ID: 2e636d5e66c35dfcbaf617aa8fa963f6847478fe Gitweb: http://git.kernel.org/tip/2e636d5e66c35dfcbaf617aa8fa963f6847478fe Author: Peter Zijlstra <peterz@infradead.org> AuthorDate: Mon, 16 May 2016 15:01:11 +0200 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Tue, 17 May 2016 12:24:21 +0200
sched/preempt: Fix preempt_count manipulations
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> --- include/asm-generic/preempt.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h index 5d8ffa3..c1cde35 100644 --- 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 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 ¤t_thread_info()->preempt_count; }
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