Re: [PATCH] task_work_run: don't take ->pi_lock unconditionally

Oleg Nesterov
2020-02-21T14:53:04Z

On 02/20, Peter Zijlstra wrote:
>
> On Thu, Feb 20, 2020 at 06:22:02PM +0100, Oleg Nesterov wrote:
> > @@ -68,10 +65,10 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
> > * we raced with task_work_run(), *pprev == NULL/exited.
> > */
> > raw_spin_lock_irqsave(&task->pi_lock, flags);
> > + for (work = READ_ONCE(*pprev); work; ) {
> > if (work->func != func)
> > pprev = &work->next;
>
> But didn't you loose the READ_ONCE() of *pprev in this branch?

Argh, yes ;)

> > @@ -97,16 +94,16 @@ void task_work_run(void)
> > * work->func() can do task_work_add(), do not set
> > * work_exited unless the list is empty.
> > */
> > + work = READ_ONCE(task->task_works);
> > do {
> > head = NULL;
> > if (!work) {
> > if (task->flags & PF_EXITING)
> > head = &work_exited;
> > else
> > break;
> > }
> > + } while (!try_cmpxchg(&task->task_works, &work, head));
> >
> > if (!work)
> > break;
>
> But given that, as you say, cancel() could have gone and stole our head,
> should we not try and install &work_exiting when PF_EXITING in that
> case?

cancel() can't do this, as long as we use cmpxchg/try_cmpxchg, not xchg().
This is what the comment before lock/unlock below tries to explain.

> That is; should we not do continue in that case, instead of break.

This is what we should do if we use xchg() like your previous version did.
Or I am totally confused. Hmm, and when I re-read my words it looks as if
I am trying to confuse you.

So lets "simplify" this code assuming that PF_EXITING is set:

work = READ_ONCE(task->task_works);
do {
head = NULL;
if (!work)
head = &work_exited;
} while (!try_cmpxchg(&task->task_works, &work, head));

if (!work)
break;

If work == NULL after try_cmpxchg() _succeeds_, then the new "head" must
be work_exited and we have nothing to do.

If it was nullified by try_cmpxchg(&work) because we raced with cancel_(),
then this try_cmpxchg() should have been failed.

Right?

> @@ -69,9 +68,12 @@ task_work_cancel(struct task_struct *tas
> */
> raw_spin_lock_irqsave(&task->pi_lock, flags);
> while ((work = READ_ONCE(*pprev))) {
> - if (work->func != func)
> + if (work->func != func) {
> pprev = &work->next;
> - else if (cmpxchg(pprev, work, work->next) == work)
> + continue;
> + }
> +
> + if (try_cmpxchg(pprev, &work, work->next))
> break;

perhaps I misread this code, but it looks a bit strange to me... it doesn't
differ from

while ((work = READ_ONCE(*pprev))) {
if (work->func != func)
pprev = &work->next;
else if (try_cmpxchg(pprev, &work, work->next))
break;
}

either way it is correct, the only problem is that we do not need (want)
another READ_ONCE() if try_cmpxchg() fails.

> void task_work_run(void)
> {
> struct task_struct *task = current;
> - struct callback_head *work, *head, *next;
> + struct callback_head *work, *next;
>
> for (;;) {
> - /*
> - * work->func() can do task_work_add(), do not set
> - * work_exited unless the list is empty.
> - */
> - do {
> - head = NULL;
> - work = READ_ONCE(task->task_works);
> - if (!work) {
> - if (task->flags & PF_EXITING)
> - head = &work_exited;
> - else
> - break;
> - }
> - } while (cmpxchg(&task->task_works, work, head) != work);
> + work = READ_ONCE(task->task_works);
> + if (!work) {
> + if (!(task->flags & PF_EXITING))
> + return;
> +
> + /*
> + * work->func() can do task_work_add(), do not set
> + * work_exited unless the list is empty.
> + */
> + if (try_cmpxchg(&task->task_works, &work, &work_exited))
> + return;
> + }
> +
> + work = xchg(&task->task_works, NULL);
> + if (!work)
> + continue;

looks correct...

Oleg.