From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marco Stornelli <marco.stornelli@gmail.com>
Subject: Re: [PATCH RFC] Btrfs: fix deadlock between sys_sync and freeze
Date: Tue, 14 Aug 2012 14:59:30 +0200
Message-ID: <502A4BB2.6000404@gmail.com>
References: <1344920474-10014-1-git-send-email-liub.liubo@gmail.com>
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Cc: linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
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Il 14/08/2012 07:01, liub.liubo@gmail.com ha scritto:
> From: Liu Bo <bo.li.liu@oracle.com>
>
> I found this while testing xfstests 068, the story is
>
>      t1                                            t2
>    sys_sync                                    thaw_super
>      iterate_supers
>        down_read(sb->s_umount)                   down_write(sb->s_umount) --->wait for t1
>        sync_fs (with wait mode)
>          start_transaction
>            sb_start_intwrite --------------------> wait for t2 to set s_writers.frozen to SB_UNFROZEN
>
> In this patch, I add an helper sb_start_intwrite_trylock() and use it before we
> start_transaction in sync_fs() with wait mode so that we won't hit the deadlock.
>

IMHO, we should avoid to call the sync operation on a frozen fs. The 
freeze operation, indeed, already include a sync operation. According to 
man page, no other operation should modify the fs after the freeze. So 
for me the modification is inside sync_filesystem (and sync_one_sb).

Marco