linux-kernel - Re: [RFC][PATCH] signals: don't copy siginfo

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Message-ID: <20090226203653.GA9285@redhat.com>
Date:	Thu, 26 Feb 2009 21:36:53 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Vegard Nossum <vegard.nossum@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] signals: don't copy siginfo_t on dequeue

On 02/26, Vegard Nossum wrote:
>
> 2009/2/26 Oleg Nesterov <oleg@...hat.com>:
> > So. dequeue_signal() returns NULL if there is no siginfo queued. In that
> > case we assume that the signal is not pending.
> >
> > But this is not right. Think about SEND_SIG_FORCED, or __sigqueue_alloc()
> > failure when the signal is sent. Or look at zap_other_threads() for example,
> > it just sets the bit in ->pending but doesn't queue siginfo.
>
> I will investigate.

Cough. Well, I must admit I am a bit skeptical about this patch ;) Because
I suspect it will add more complications to the code. And _I think_ avoiding
copy_siginfo() does not buy too much. I will be happy if I am wrong, though.

But. If you are going to do another version, then please note there is another
problem with this patch, SIGQUEUE_PREALLOC.

If collect_signal() returns SIGQUEUE_PREALLOC info, we can not drop ->siglock.
I mean, once we drop ->siglock, this info can be freed, so for example

			spin_unlock(&tsk->sighand->siglock);
	-               do_schedule_next_timer(info);
	+               do_schedule_next_timer(&signal->info);

even this part is not safe.

Also. The patch uses __sigqueue_free() to free the delivered siginfo, but
this is not safe without ->siglock, we can race with sigqueue_free().

Oleg.

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