linux-ext4 - Re: [PATCH] ext4: Fix deadlock with fs freezing and EA inodes

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Message-ID: <X9mS0Vz5UyK79cqp@mit.edu>
Date:   Tue, 15 Dec 2020 23:53:37 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, stable@...r.kernel.org,
        Tahsin Erdogan <tahsin@...gle.com>
Subject: Re: [PATCH] ext4: Fix deadlock with fs freezing and EA inodes

On Fri, Nov 27, 2020 at 12:06:49PM +0100, Jan Kara wrote:
> Xattr code using inodes with large xattr data can end up dropping last
> inode reference (and thus deleting the inode) from places like
> ext4_xattr_set_entry(). That function is called with transaction started
> and so ext4_evict_inode() can deadlock against fs freezing like:
> 
> CPU1					CPU2
> 
> removexattr()				freeze_super()
>   vfs_removexattr()
>     ext4_xattr_set()
>       handle = ext4_journal_start()
>       ...
>       ext4_xattr_set_entry()
>         iput(old_ea_inode)
>           ext4_evict_inode(old_ea_inode)
> 					  sb->s_writers.frozen = SB_FREEZE_FS;
> 					  sb_wait_write(sb, SB_FREEZE_FS);
> 					  ext4_freeze()
> 					    jbd2_journal_lock_updates()
> 					      -> blocks waiting for all
> 					         handles to stop
>             sb_start_intwrite()
> 	      -> blocks as sb is already in SB_FREEZE_FS state
> 
> Generally it is advisable to delete inodes from a separate transaction
> as it can consume quite some credits however in this case it would be
> quite clumsy and furthermore the credits for inode deletion are quite
> limited and already accounted for. So just tweak ext4_evict_inode() to
> avoid freeze protection if we have transaction already started and thus
> it is not really needed anyway.
> 
> CC: stable@...r.kernel.org
> Fixes: dec214d00e0d ("ext4: xattr inode deduplication")
> CC: Tahsin Erdogan <tahsin@...gle.com>
> Signed-off-by: Jan Kara <jack@...e.cz>

thanks, applied.

					- Ted