linux-kernel - Re: [PATCH 3/3] posix timers: use SIGQUEUE

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Date:	Sat, 17 May 2008 10:11:37 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	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>,
	Roland McGrath <roland@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] posix timers: use SIGQUEUE_CANCELLED when the timer
 is destroyed

On Sat, 17 May 2008, Oleg Nesterov wrote:

> On 05/17, Oleg Nesterov wrote:
> >
> > This is a user visible change. With this patch sys_timer_delete() discards
> > the pending signal which was generated by the timer.
> 
> If this change is undesirable, we can (for example) do
> 
> 	--- kernel/posix-timers.c
> 	+++ kernel/posix-timers.c
> 	@@ -885,6 +885,7 @@ itimer_delete(struct k_itimer *timer)
> 		timer->it_process = NULL;
> 	 
> 		unlock_timer(timer, flags);
> 	+	tmr->sigq->flags |= SIGQUEUE_CANCELLED;
> 		release_posix_timer(timer, IT_ID_SET);
> 	 }
> 
> instead, and still fix the "BUG 10460".

The only reason I like that better is that it makes me nervous when one 
re-initializes the whole flags field. So your original 3/3 patch

	-       q->flags &= ~SIGQUEUE_PREALLOC;
	+       q->flags = SIGQUEUE_CANCELLED; /* clears SIGQUEUE_PREALLOC */

just makes me go "Hmm, what about all the other flag bits?"

Now, admittedly, there are currently (with your patch) just two 
SIGQUEUE_xyz bits, so by just doing that single assignment, you really 
only modify the two bits you want to modify. But maybe that will change. 
So I'd prefer to either write it as

	q->flags &= ~SIGQUEUE_PREALLOC;
	q->flags |= SIGQUEUE_CANCELLED;

or to use bitfields, or to do something else to make it safe in the 
presense of multiple bits.

Your alternate patch obviously doesn't have that issue, since it just sets 
the single bit.

But apart from that issue, I have absolutely no preferences either way. 
You're effectively the maintainer in this area, you get to choose.

		Linus
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