linux-ext4 - Re: [PATCH] ext4: take i_mutex in ext4_symlink to eliminate a warning from ext4

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Message-ID: <20130328140635.GA32635@thunk.org>
Date:	Thu, 28 Mar 2013 10:06:35 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Zheng Liu <wenqing.lz@...bao.com>,
	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ext4: take i_mutex in ext4_symlink to eliminate a
 warning from ext4_truncate

I looked more closely at the assumption that ext4_write_begin() holds
i_mutex.  This is guaranteed by Documentation/filesystems/Locking,
which notes that write_begin() and write_end() functions hold i_mutex:

			PageLocked(page)	i_mutex
write_begin:		locks the page		yes
write_end:		yes, unlocks		yes

So the bug is that ext4_symlink() calls __page_symlink();
__page_symlink() calls pagecache_write_begin() which calls
write_begin(), without taking i_mutex.

So we can fix this by taking i_mutex in ext4_symlink(), but I think it
would be better to take the i_mutex in __page_symlink(), since it
would then address a violation of the locking rules for all file
systems.

Al, do you agree?

					- Ted
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