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Date: Fri, 13 May 2016 16:58:14 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Vikram Mulukutla <markivx@...eaurora.org> Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org Subject: Re: Additional compiler barrier required in sched_preempt_enable_no_resched? On Thu, May 12, 2016 at 11:39:47PM -0700, Vikram Mulukutla wrote: > Hi, > > I came across a piece of engineering code that looked like: > > preempt_disable(); > /* --cut, lots of code-- */ > preempt_enable_no_resched(); > put_user() > preempt_disable(); > > (If you wish to seriously question the usage of the preempt API in this > manner, I unfortunately have no comment since I didn't write the code.) I'm with Thomas here, that's broken and should not be done. > This particular block of code was causing lockups and crashes on a certain > ARM64 device. The generated assembly revealed that the compiler was simply > optimizing out the increment and decrement of the preempt count, allowing > put_user to run without preemption enabled, causing all sorts of badness. > Since put_user doesn't actually access the preempt count and translates to > just a few instructions without any branching, I suppose that the compiler > figured it was OK to optimize. > > The immediate solution is to add a compiler barrier to the code above, but > should sched_preempt_enable_no_resched have an additional compiler barrier > after (has one before already) the preempt-count decrement to prevent this > sort of thing? I think the below would be sufficient; IIRC the compiler may not combine or elide volatile operations. --- 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 5d8ffa3e6f8c..c1cde3577551 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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