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SubjectRe: [PATCH 08/11] membarrier: Provide core serializing command (v2)
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.

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

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