linux-kernel - Re: [PATCH] sigqueue_free: fix the race with collect

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Message-ID: <20070824110836.GA74@tv-sign.ru>
Date:	Fri, 24 Aug 2007 15:08:36 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	taoyue <yue.tao@...driver.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...ru>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH] sigqueue_free: fix the race with collect_signal()

On 08/24, taoyue wrote:
>
> Oleg Nesterov wrote:
> >>
> >>collect_signal:				sigqueue_free:
> >>
> >>	list_del_init(&first->list);
> >>                                       spin_lock_irqsave(lock, flags);
> >>    
> >                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >  
> >>                                       if (!list_empty(&q->list))
> >>                                             list_del_init(&q->list);
> >>                                       spin_unlock_irqrestore(lock, 
> >>                                       flags);
> >>                                       q->flags &= ~SIGQUEUE_PREALLOC;
> >>
> >>       __sigqueue_free(first);		__sigqueue_free(q);
> >>    
> >
> >collect_signal() is always called under ->siglock which is also taken by
> >sigqueue_free(), so this is not possible.
> >
> >Basically, this patch is the same one-liner I sent you before
> >
> >	http://marc.info/?l=linux-kernel&m=118772206603453&w=2
> >
> >(Thanks for the additional testing and report, btw).
> >
> >P.S. It would be nice to know if this patch solves the problems reported
> >by Jeremy, but his email is disabled.
> >
> >Oleg.
> >
> >  
> I know, using current->sighand->siglock to prevent one sigqueue
> is free twice. I want to know whether it is possible that the two
> function is called in different thread. If that, the spin_lock is useless.

Not sure I understand. Yes, it is possible they are called by 2 different
threads, that is why we had a race. But all threads in the same thread
group have the same ->sighand, and thus the same ->sighand->siglock.

Oleg.

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