linux-kernel - Re: [RFC PATCH v2] posix-timers: Support delivery of signals to the current thread

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Date:   Wed, 25 Jan 2023 17:31:37 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: [RFC PATCH v2] posix-timers: Support delivery of signals to the
 current thread

On 01/25, Dmitry Vyukov wrote:
>
> > diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> > index 5dead89308b7..e38b53a0f814 100644
> > --- a/kernel/time/posix-timers.c
> > +++ b/kernel/time/posix-timers.c
> > @@ -336,6 +336,7 @@ void posixtimer_rearm(struct kernel_siginfo *info)
> >  int posix_timer_event(struct k_itimer *timr, int si_private)
> >  {
> >         enum pid_type type;
> > +       struct pid *pid;
> >         int ret;
> >         /*
> >          * FIXME: if ->sigq is queued we can race with
> > @@ -350,8 +351,9 @@ int posix_timer_event(struct k_itimer *timr, int si_private)
> >          */
> >         timr->sigq->info.si_sys_private = si_private;
> >
> > -       type = !(timr->it_sigev_notify & SIGEV_THREAD_ID) ? PIDTYPE_TGID : PIDTYPE_PID;
> > -       ret = send_sigqueue(timr->sigq, timr->it_pid, type);
> > +       type = (timr->it_sigev_notify & SIGEV_THREAD_ID) ? PIDTYPE_PID : PIDTYPE_TGID;
> > +       pid = (type == PIDTYPE_PID) ? timr->it_pid : task_pid(current);
> > +       ret = send_sigqueue(timr->sigq, pid, type);
> >         /* If we failed to send the signal the timer stops. */
> >         return ret > 0;
> >  }
>
> Hi Oleg,
>
> This is indeed much simpler!
>
> Do I understand correctly that:
> 1. I would need to use SIGEV_SIGNAL (without SIGEV_THREAD_ID)

Yes,

> 2. The signal is still queued into process shared_pending

Yes. But just in case, please note that if this signal is not realtime
(sigev_signo < SIGRTMIN) and it is already queued, it will be dropped.
And I do not know if this can work for you.

However this is what we already have with SIGEV_SIGNAL w/o SIGEV_THREAD_ID,
and the same is true for SIGEV_THREAD_ID if the signal is already pending in
target_task->pending.

> 3. If the current task has not blocked the signal (it shouldn't), then
> it won't kick any other task

Yes,

> 4. The current task will likely deliver the signal right on the timer
> interrupt return to userspace
> ?

Yes.

But! I just noticed send_sigqueue() does pid_task(pid, type), so the patch
above needs another change


	--- a/kernel/signal.c
	+++ b/kernel/signal.c
	@@ -1970,7 +1970,8 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
	 
		ret = -1;
		rcu_read_lock();
	-	t = pid_task(pid, type);
	+	// comment to explain why don't we use "type"
	+	t = pid_task(pid, PIDTYPE_PID);
		if (!t || !likely(lock_task_sighand(t, &flags)))
			goto ret;
	 


> This changes the existing behavior (the "average bear" may be surprised :))
> https://elixir.bootlin.com/linux/v6.2-rc5/source/kernel/signal.c#L1007

this comment looks a bit misleading, s/main thread/target thread/

> But currnently it's also queued into shared_pending and any thread
> could get the signal anyway. So I think this should be fine.

Yes.

> On the positive side: it should improve performance. Delivering to the
> currently running task is better on all fronts (no kicking,
> rescheduling, IPIs, better locality), right?

Well, iiuc this was the goal of your patch ? ;)

Oleg.