linux-kernel - Re: [PATCH 1/3] signals: sigqueue_free: don't free sigqueue if it is queued

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Date:	Thu, 22 May 2008 15:12:11 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Austin Clements <amdragon+kernelbugzilla@....edu>,
	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] signals: sigqueue_free: don't free sigqueue if it is queued

On 05/21, Roland McGrath wrote:
>
> -void flush_sigqueue(struct sigpending *queue)
> +static void __flush_sigqueue(struct sigpending *queue, int timers)
>  {
>  	struct sigqueue *q;
>  
>  	sigemptyset(&queue->signal);
>  	while (!list_empty(&queue->list)) {
>  		q = list_entry(queue->list.next, struct sigqueue , list);
> +		if (timers && q->info.si_code != SI_TIMER)
> +			continue;
>  		list_del_init(&q->list);
>  		__sigqueue_free(q);
>  	}
>  }

This is not enough. Again, we remove and free sigqueue but don't discard
the pending signal. (and we must take into account other rt signals with
the same si_signo if we want to discard the signal).

Oh, this problem is unexpectedly nasty. It is trivial and minor, we can
solve it in may ways, but personally I can't find a simple/clean way.

Let's look at my first attempt,

	http://marc.info/?l=linux-kernel&m=120888210417700

the patch was "almost" correct.
We can add the "bool cancel" parameter to sigqueue_free(), true when
called from exec (or exit_itimers). In that case SIGQUEUE_SHARED_PENDING
is enough: the pending signal was either sent to current, or it is group
wide. Not nice too of course, but afaics a bit simpler. Actually, the
patch exists: http://marc.info/?l=linux-kernel&m=120888210417698

What do you think?

(instead of SIGQUEUE_SHARED_PENDING, we can encode "struct sigpending *"
 in q->flags, but this is really awful and I agree with Linus on the
 EINTR/etc issues).

I'll try to think more on Weekend.

Oleg.

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