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

[Mathieu Desnoyers]

----- On Jan 29, 2018, at 1:15 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx 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