Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier
From: Mathieu Desnoyers
Date: Tue Aug 04 2020 - 13:25:26 EST
----- On Aug 4, 2020, at 12:51 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
> 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.
I am not against using this already existing barrier to provide the
guarantee we need, but it would have to be documented in the code.