Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Sep 4, 2019, at 12:09 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Wed, Sep 04, 2019 at 11:19:00AM -0400, Mathieu Desnoyers wrote:
>> ----- On Sep 3, 2019, at 4:36 PM, Linus Torvalds torvalds@linux-foundation.org
>> wrote:
> 
>> > I wonder if the easiest model might be to just use a percpu variable
>> > instead for the membarrier stuff? It's not like it has to be in
>> > 'struct task_struct' at all, I think. We only care about the current
>> > runqueues, and those are percpu anyway.
>> 
>> One issue here is that membarrier iterates over all runqueues without
>> grabbing any runqueue lock. If we copy that state from mm to rq on
>> sched switch prepare, we would need to ensure we have the proper
>> memory barriers between:
>> 
>> prior user-space memory accesses  /  setting the runqueue membarrier state
>> 
>> and
>> 
>> setting the runqueue membarrier state / following user-space memory accesses
>> 
>> Copying the membarrier state into the task struct leverages the fact that
>> we have documented and guaranteed those barriers around the rq->curr update
>> in the scheduler.
> 
> Should be the same as the barriers we already rely on for rq->curr, no?
> That is, if we put this before switch_mm() then we have
> smp_mb__after_spinlock() and switch_mm() itself.

Yes, I think we can piggy-back on the already documented barriers documented around
rq->curr store.

> Also, if we place mm->membarrier_state in the same cacheline as mm->pgd
> (which switch_mm() is bound to load) then we should be fine, I think.

Yes, if we make sure membarrier_prepare_task_switch only updates the
rq->membarrier_state if prev->mm != next->mm, we should be able to avoid
loading next->mm->membarrier_state when switch_mm() is not invoked.

I'll prepare RFC patch implementing this approach.

Thanks,

Mathieu


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