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

[Peter Zijlstra <peterz@infradead.org>]

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.

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