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Additional compiler barrier required in sched_preempt_enable_no_resched?

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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.)

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?

Thanks,
Vikram

--
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora 
Forum, a Linux Foundation Collaborative Project

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Additional compiler barrier required in  sched_preempt_enable_no_resched? Vikram Mulukutla <markivx@codeaurora.org> - 2016-05-13 08:50 +0200
  Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Thomas Gleixner <tglx@linutronix.de> - 2016-05-13 16:30 +0200
  Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Peter Zijlstra <peterz@infradead.org> - 2016-05-13 17:00 +0200
    Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Vikram Mulukutla <markivx@codeaurora.org> - 2016-05-14 00:50 +0200
      Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Thomas Gleixner <tglx@linutronix.de> - 2016-05-14 17:50 +0200
        Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Vikram Mulukutla <markivx@codeaurora.org> - 2016-05-14 20:30 +0200
        Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Peter Zijlstra <peterz@infradead.org> - 2016-05-16 13:00 +0200
          Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Peter Zijlstra <peterz@infradead.org> - 2016-05-16 13:10 +0200
          Re: Additional compiler barrier required in  sched_preempt_enable_no_resched? Peter Zijlstra <peterz@infradead.org> - 2016-05-16 15:20 +0200
            [tip:sched/urgent] sched/preempt: Fix preempt_count manipulations tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-05-17 16:30 +0200

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