On 3/12/21 2:24 PM, Linus Torvalds wrote:
> On Fri, Mar 12, 2021 at 1:17 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> I'm _guessing_ it's just that now those threads are user threads, and
>> then the freezing logic expects them to freeze/thaw using a signal
>> machinery or something like that. And that doesn't work when there is
>> no signal handling for those threads.
>
> IOW, I think it's this logic in freeze_task():
>
> if (!(p->flags & PF_KTHREAD))
> fake_signal_wake_up(p);
> else
> wake_up_state(p, TASK_INTERRUPTIBLE);
>
> where that "not a PF_KTHREAD" test will trigger for the io_uring
> threads, and it does that fake_signal_wake_up(), and then
> signal_wake_up() does that
>
> set_tsk_thread_flag(t, TIF_SIGPENDING);
>
> but the io_uring thread has no way to react to it.
>
> So now the io_uring thread will see "I have pending signals" (even if
> there is no actual pending signal - it's just a way to get normal
> processes to get out of TASK_INTERRUPTIBLE and return to user space
> handling).
>
> And the pending fake signal will never go away, because there is no
> "return to user space" handling.
>
> So I think the fix is to simply make freeze_task() not do that fake
> signal thing for IO-uring threads either.
Ah good catch, I missed it. Yes I bet this is exactly what it is, and
it just needs the one liner doing:
if (!(p->flags & (PF_IO_WORKER | PF_KTHREAD)))
fake_signal_wake_up(p);
else
wake_up_state(p, TASK_INTERRUPTIBLE);
instead. I'll try this out and test it, then we can drop the PF_NOFREEZE
business going forward. Thanks!
--
Jens Axboe
