linux-ext4 - Re: BUG? a suspected deadlock bug at ext4_fill

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Message-ID: <20090730180241.GC9223@atrey.karlin.mff.cuni.cz>
Date:	Thu, 30 Jul 2009 20:02:41 +0200
From:	Jan Kara <jack@...e.cz>
To:	홍신 shin hong <hongshin@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: BUG? a suspected deadlock bug at ext4_fill_super()

  Hello,

> Hi. I found a suspected deadlock bug from ext4.
> 
> ext4_fill_super() releases and re-takes BKL while s_umount is held.
> Lock ordering at the point where BKL is re-taken is s_umount-> BKL.
> 
> However, at do_remount(), locking ordering is BKL->s_umount.
> Therefore, concurrent execution of do_remount() and ext4_fill_super()
> may result deadlock.
  Hmm, I don't see how this could deadlock since we seem to take kernel
lock only in do_new_mount() before calling do_kern_mount(). But you are
right that taking the kernel lock with s_umount_sem looks fishy...

> p.s. I found this suspected bug by the motivation from
>        commit 5f22ca9b13551debea77a407a8d06cd9c6f15238.
  From a quick look, this looks like a different matter - they have used
lock_super() instead of lock_kernel() and that's used in more places.

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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