linux-kernel - Re: [patch V5 09/26] posix-timers: Make signal overrun accounting sensible

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Message-ID: <ZxbNlw4F5tUI1a5D@pavilion.home>
Date: Mon, 21 Oct 2024 23:54:31 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Stephen Boyd <sboyd@...nel.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [patch V5 09/26] posix-timers: Make signal overrun accounting
 sensible

Le Tue, Oct 01, 2024 at 10:42:12AM +0200, Thomas Gleixner a écrit :
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> The handling of the timer overrun in the signal code is inconsistent as it
> takes previous overruns into account. This is just wrong as after the
> reprogramming of a timer the overrun count starts over from a clean state,
> i.e. 0.
> 
> Make the accounting in send_sigqueue() consistent with that.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/signal.c |   34 ++++++++++++++++++++++++++++------
>  1 file changed, 28 insertions(+), 6 deletions(-)
> ---
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1961,6 +1961,34 @@ int send_sigqueue(struct sigqueue *q, st
>  	 */
>  	q->info.si_sys_private = si_private;
>  
> +	/*
> +	 * Set the overrun count to zero unconditionally. The posix timer
> +	 * code does not self rearm periodic timers. They are rearmed from
> +	 * dequeue_signal().
> +	 *
> +	 * But there is a situation where @q is already enqueued:
> +	 *
> +	 * 1) timer_settime()
> +	 *      arm_timer()
> +	 * 2) timer_expires()
> +	 *      send_sigqueue(@q)
> +	 *        enqueue(@q)
> +	 * 3) timer_settime()
> +	 *      arm_timer()
> +	 * 4) timer_expires()
> +	 *      send_sigqueue(@q) <- Observes @q already queued
> +	 *
> +	 * In this case incrementing si_overrun does not make sense because
> +	 * there is no relationship between timer_settime() #1 and #2.
> +	 *
> +	 * The POSIX specification is useful as always: "The effect of
> +	 * disarming or resetting a timer with pending expiration
> +	 * notifications is unspecified."
> +	 *
> +	 * Just do the sensible thing and reset the overrun.
> +	 */
> +	q->info.si_overrun = 0;

So this means that in the above example case, no signal at all is going to be
delivered (because the seq will be impaired on the previously queued
signal) and no overrun count will be incremented either?

> +
>  	ret = 1; /* the signal is ignored */
>  	result = TRACE_SIGNAL_IGNORED;
>  	if (!prepare_signal(sig, t, false))
> @@ -1968,15 +1996,9 @@ int send_sigqueue(struct sigqueue *q, st
>  
>  	ret = 0;
>  	if (unlikely(!list_empty(&q->list))) {
> -		/*
> -		 * If an SI_TIMER entry is already queue just increment
> -		 * the overrun count.
> -		 */
> -		q->info.si_overrun++;

Who is ever incrementing this after that? I'm a bit confused between the
timer overrun and the sigqueue overrun. Those seem to be two different
things without any link...

Thanks.

>  		result = TRACE_SIGNAL_ALREADY_PENDING;
>  		goto out;
>  	}
> -	q->info.si_overrun = 0;
>  
>  	signalfd_notify(t, sig);
>  	pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
>