Additional compiler barrier required in sched_preempt_enable_no_resched?

[Vikram Mulukutla <markivx@codeaurora.org>]

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

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