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Re: 3.14-rc2 XFS backtrace because irqs_disabled.

To: Oleg Nesterov <oleg@xxxxxxxxxx>
Subject: Re: 3.14-rc2 XFS backtrace because irqs_disabled.
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Sat, 15 Feb 2014 15:36:31 +0000
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Dave Jones <davej@xxxxxxxxxx>, Eric Sandeen <sandeen@xxxxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140215152251.GY18016@xxxxxxxxxxxxxxxxxx>
References: <CA+55aFxy2t7bnCUc-DhhxYxsZ0+GwL9GuQXRYtE_VzqZusmB9A@xxxxxxxxxxxxxx> <20140212113928.GO18016@xxxxxxxxxxxxxxxxxx> <CA+55aFywwx0Q8xK2GJiRJ+FV7PQEKoBRxDUxW4052FVyd5XOpg@xxxxxxxxxxxxxx> <20140212211421.GP18016@xxxxxxxxxxxxxxxxxx> <CA+55aFyobyUNFo=3rpdbxTqgV7OQetCKbCfwEEbgxUcT-1+30w@xxxxxxxxxxxxxx> <20140213174020.GA14455@xxxxxxxxxx> <CA+55aFxwozCQ05axLB02R3huX8sj=20EoFfw0cSDDL8fBE_Y6Q@xxxxxxxxxxxxxx> <20140215052531.GX18016@xxxxxxxxxxxxxxxxxx> <20140215142700.GA15540@xxxxxxxxxx> <20140215152251.GY18016@xxxxxxxxxxxxxxxxxx>
Sender: Al Viro <viro@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Sat, Feb 15, 2014 at 03:22:51PM +0000, Al Viro wrote:
> On Sat, Feb 15, 2014 at 03:27:00PM +0100, Oleg Nesterov wrote:
> 
> > 1. info->q can be already freed if SIGQUEUE_PREALLOC.
> > 
> >    Once get_signal_to_deliver() or any other caller drops ->siglock
> >    another thread can do sys_timer_delete()->sigqueue_free().
> 
> How the devil would it find the sucker?  It's off the list already.

Ouch...  I think I see what you mean.  Let me see if I got it right:
timer->sigq is *not* freed by collect_signal(); it's done by
release_posix_timer() instead, under siglock.  Frankly, this
        /*
         * If it is queued it will be freed when dequeued,
         * like the "regular" sigqueue.
         */
        if (!list_empty(&q->list))
                q = NULL;
in sigqueue_free() smells like it's asking for races.  Sigh...

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