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Date: Tue, 29 Oct 2024 21:18:40 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	John Stultz <jstultz@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Stephen Boyd <sboyd@kernel.org>,
	Eric Biederman <ebiederm@xmission.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [patch V5 16/26] signal: Replace resched_timer logic
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In-Reply-To: <875xpa1zzo.ffs@tglx>

Le Tue, Oct 29, 2024 at 08:16:27PM +0100, Thomas Gleixner a écrit :
> On Tue, Oct 29 2024 at 18:55, Frederic Weisbecker wrote:
> > Le Tue, Oct 29, 2024 at 05:55:38PM +0100, Thomas Gleixner a écrit :
> >> It still happens because SIGQUEUE_PREALLOC is cleared in sigqueue_free()
> >> 
> >> __sigqueue_free() has
> >>        if (q->flags & PREALLOC)
> >>        	     return;
> >> 
> >> So the old code called __sigqueue_free() unconditionally which just
> >> returned. But now we have a condition to that effect already, so why
> >> call into __sigqueue_free() for nothing?
> >
> > 1) Signal is queued
> > 2) Timer is deleted, sigqueue() clears SIGQUEUE_PREALLOC but doesn't go
> >    further because the sigqueue is queued
> > 3) Signal is collected and delivered but it's not calling __sigqueue_free()
> >    so the sigqueue is not released.
> >
> > This is "fixed" on the subsequent patch which uses embedded sigqueue and
> > rcuref but this patch alone breaks.
> >
> > Or am I missing something that prevents it?
> 
> Again:
> 
> > 1) Signal is queued
> > 2) Timer is deleted, sigqueue() clears SIGQUEUE_PREALLOC but doesn't go
> >    further because the sigqueue is queued
> 
>   3)
>   
> void collect_signal(..)
> 
>      if (unlikely((first->flags & SIGQUEUE_PREALLOC) && (info->si_code == SI_TIMER)))
> 	*timer_sigq = first;    // Path NOT taken because SIGQUEUE_PREALLOC is not set
>      else
>      	__sigqueue_free(first); // Path taken and frees it
> 
> No?

Duh! Yes of course...

Thanks.

> 
> Thanks,
> 
>         tglx