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From:   Andreas Dilger <adilger@dilger.ca>
Message-Id: <96677D90-C895-4356-83A9-A116A91F199A@dilger.ca>
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Subject: Re: [PATCH] ext4: Fix deadlock due to mbcache entry corruption
Date:   Mon, 5 Dec 2022 18:52:34 -0700
In-Reply-To: <9c414060-989d-55bb-9a7b-0f33bf103c4f@leemhuis.info>
Cc:     linux-ext4@vger.kernel.org, stable@vger.kernel.org,
        Thilo Fromm <t-lo@linux.microsoft.com>,
        Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Thorsten Leemhuis <regressions@leemhuis.info>
To:     Jan Kara <jack@suse.cz>, Ted Tso <tytso@mit.edu>
References: <20221123193950.16758-1-jack@suse.cz>
 <20221201151021.GA18380@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
 <9c414060-989d-55bb-9a7b-0f33bf103c4f@leemhuis.info>
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On Wed, Nov 23, 2022 at 08:39:50PM +0100, Jan Kara wrote:
> When manipulating xattr blocks, we can deadlock infinitely looping
> inside ext4_xattr_block_set() where we constantly keep finding xattr
> block for reuse in mbcache but we are unable to reuse it because its
> reference count is too big. This happens because cache entry for the
> xattr block is marked as reusable (e_reusable set) although its
> reference count is too big. When this inconsistency happens, this
> inconsistent state is kept indefinitely and so ext4_xattr_block_set()
> keeps retrying indefinitely.
>=20
> The inconsistent state is caused by non-atomic update of e_reusable =
bit.
> e_reusable is part of a bitfield and e_reusable update can race with
> update of e_referenced bit in the same bitfield resulting in loss of =
one
> of the updates. Fix the problem by using atomic bitops instead.
>=20
> CC: stable@vger.kernel.org
> Fixes: 6048c64b2609 ("mbcache: add reusable flag to cache entries")
> Reported-and-tested-by: Jeremi Piotrowski =
<jpiotrowski@linux.microsoft.com>
> Reported-by: Thilo Fromm <t-lo@linux.microsoft.com>
> Link: =
https://lore.kernel.org/r/c77bf00f-4618-7149-56f1-b8d1664b9d07@linux.micro=
soft.com/
> Signed-off-by: Jan Kara <jack@suse.cz>

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Cheers, Andreas


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