linux-kernel - Re: [patch V5 16/26] signal: Replace resched

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Message-ID: <875xpa1zzo.ffs@tglx>
Date: Tue, 29 Oct 2024 20:16:27 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
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 16/26] signal: Replace resched_timer logic

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?

Thanks,

        tglx