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

Jens Axboe
2020-02-28T19:18:03Z

On 2/24/20 11:47 AM, Jens Axboe wrote:
> 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.

Peter/Oleg, gentle ping on this query. I'd like to queue up the task poll
rework on the io_uring side, but I still have this one at the start of
the series:

https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-task-poll&id=3b668ecf75f94f40c1faf9688ba3f32fb5e9f5d0

--
Jens Axboe