Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier

[Mathieu Desnoyers]

----- 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
>> barrier()
>> y = 1
>> exit()
>> exit_mm()
>> current->mm = NULL;
>>                         r1 = load y
>>                         membarrier()
>>                           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.

Good point.

> 
> 
> 
> 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.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com