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

Jens Axboe
2020-02-24T18:47:30Z

On 2/21/20 7:52 AM, Oleg Nesterov wrote:
> 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...

Peter/Oleg, as you've probably noticed, I'm still hauling Oleg's
original patch around. Is the above going to turn into a separate patch
on top? If so, feel free to shove it my way as well for some extra
testing.

--
Jens Axboe