On Mon, Jul 30, 2018 at 10:30:11AM -0400, Rik van Riel wrote:
> > What happened to the rework I did there? That not only avoided
> > fiddling
> > with active_mm, but also avoids grab/drop cycles for the other
> > architectures when doing task->kthread->kthread->task things.
>
> I don't think I saw that. I only saw your email from
> July 20th with this fragment of code, which does not
> appear to avoid the grab/drop cycles, and still fiddles
> with active_mm:
Yeah, that's it. Note how it doesn't do a grab+drop for kernel->kernel,
where the current could would have.
And also note that it only fiddles with active_mm if it does the
grab+drop thing (the below should have s/ifdef/ifndef/ to make more
sense maybe).
So for ARCH_NO_ACTIVE_MM we never touch ->active_mm and therefore
->active_mm == ->mm.
> + /*
> + * kernel -> kernel lazy + transfer active
> + * user -> kernel lazy + mmgrab() active
> + *
> + * kernel -> user switch + mmdrop() active
> + * user -> user switch
> + */
> + if (!next->mm) { // to kernel
> + enter_lazy_tlb(prev->active_mm, next);
> +
#ifndef ARCH_NO_ACTIVE_MM
> + next->active_mm = prev->active_mm;
> + if (prev->mm) // from user
> + mmgrab(prev->active_mm);
else
prev->active_mm = NULL;
> +#endif
> + } else { // to user
> + switch_mm_irqs_off(prev->active_mm, next->mm, next);
> +
#ifndef ARCH_NO_ACTIVE_MM
> + if (!prev->mm) { // from kernel
> + /* will mmdrop() in finish_task_switch(). */
> + rq->prev_mm = prev->active_mm;
> + prev->active_mm = NULL;
> + }
> +#endif
