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Re: Additional compiler barrier required in sched_preempt_enable_no_resched?

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On Thu, 12 May 2016, Vikram Mulukutla wrote:
> 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

preempt_enable_no_resched() should not be used at all. Use preempt_enable().

Thanks,

	tglx

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Thread

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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