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Date:   Sat, 4 Sep 2021 18:11:22 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [possible bug] missed wakeup in do_sigtimedwait()?
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On Sat, Sep 04, 2021 at 10:12:09AM -0700, Linus Torvalds wrote:
> On Sat, Sep 4, 2021 at 9:59 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > I agree, that seems like a bug, and your fix seems the trivially correct thing.
> 
> Oh, never mind.  Signals are special.
> 
> Why?
> 
> Because TASK_INTERRUPTIBLE is special, and schedule() will check for
> "am I trying to sleep while a signal is pending" and will never
> actually sleep.
> 
> So you can't have missed wakeups from signals, because this sequence
> is perfectly ok, by design:
> 
>  - signal comes in and is pending
> 
>  - we set TASK_INTERRUPTIBLE
> 
>  - we are thinking about something *entirely* different, like looking
> at a pipe being emty
> 
>  - we schedule()
> 
> and the pending signal will just mean that we never go to sleep.
> 
> It's designed that way exactly so that people who have interruptible
> sleeps don't need to think about signals at all - they can concentrate
> on doing their own thing, and then do the "signal_pending()" check at
> any point without caring.

Thanks.  AFAICS, it's this logics in __schedule():
                if (signal_pending_state(prev_state, prev)) {
                        WRITE_ONCE(prev->__state, TASK_RUNNING);
IOW, TASK_INTERRUPTIBLE with signal_pending() or TASK_WAKEKILL with
pending SIGKILL.  OK...