Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier
Date: Tue Aug 04 2020 - 12:51:42 EST
On Tue, Aug 04, 2020 at 10:48:41AM -0400, Mathieu Desnoyers wrote:
> Here is the scenario I have in mind:
> Userspace variables:
> int x = 0, y = 0;
> CPU 0 CPU 1
> Thread A Thread B
> (in thread group A) (in thread group B)
> x = 1
> y = 1
> current->mm = NULL;
> r1 = load y
> skips CPU 0 (no IPI) because its current mm is NULL
> r2 = load x
> BUG_ON(r1 == 1 && r2 == 0)
Ah, yes of course.
We really should have a bunch of these scenarios in membarrier.c.
Now, the above cannot happen because we have an unconditional
atomic_dec_and_test() in do_exit() before exit_mm(), but I'm sure
relying on that is a wee bit dodgy.