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SubjectRe: [PATCH 08/11] membarrier: Provide core serializing command (v2)
----- On Jan 29, 2018, at 1:15 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Mon, Jan 29, 2018 at 07:04:14PM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 23, 2018 at 10:57:30AM -0500, Mathieu Desnoyers wrote:
>> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> > index f38c4c7e256a..041893128f51 100644
>> > --- a/kernel/sched/core.c
>> > +++ b/kernel/sched/core.c
>> > @@ -2662,9 +2662,13 @@ static struct rq *finish_task_switch(struct task_struct
>> > *prev)
>> > * thread, mmdrop()'s implicit full barrier is required by the
>> > * membarrier system call, because the current active_mm can
>> > * become the current mm without going through switch_mm().
>> > + * membarrier also requires a core serializing instruction
>> > + * before going back to user-space after storing to rq->curr.
>> > */
>> > - if (mm)
>> > + if (mm) {
>> > + membarrier_mm_sync_core_before_usermode(mm);
>> > mmdrop(mm);
>> > + }
>>
>> *confused*, when we switch from process A to process B, context_switch()
>> will not set rq->prev_mm and the above mm will be NULL and we'll not
>> pass through your_function_names_are_waaay_too_long and we'll not get
>> cookies.
>>
>> And if there's anything more complicated going on, the comment/changelog
>> are not adequate.
>
> Aaah, its the case where we do not pass through switch_mm(), the partial
> comment got to me. I only realized after reading the next patch.

Indeed, if we read the entire comment, it's made clear that this case is for
when switch_mm is not invoked, where the current mm is changed without going
through switch_mm(), when scheduling between uthread->kthread->uthread for
instance.

/*
* When transitioning from a kernel thread to a userspace
* thread, mmdrop()'s implicit full barrier is required by the
* membarrier system call, because the current active_mm can
* become the current mm without going through switch_mm().
* membarrier also requires a core serializing instruction
* before going back to user-space after storing to rq->curr.
*/
Is there something I should improve in the wording of this added
sentence to make it clearer ?

Thanks,

Mathieu


>
>> > if (unlikely(prev_state == TASK_DEAD)) {
>> > if (prev->sched_class->task_dead)
>> > prev->sched_class->task_dead(prev);
>>

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

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