linux-kernel - Re: mnt_want_write

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Message-Id: <1249324312.26977.1336.camel@nimitz>
Date:	Mon, 03 Aug 2009 11:31:52 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Nick Piggin <npiggin@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: mnt_want_write_file() has problem?

On Mon, 2009-08-03 at 06:36 +0900, OGAWA Hirofumi wrote:
> While I'm reading some code, I suspected that mnt_want_write_file() may
> have wrong assumption.  I think mnt_want_write_file() is assuming it
> increments ->mnt_writers if (file->f_mode & FMODE_WRITE). But, if it's
> special_file(), it is false?
> 
> Sorry, I'm still not checking all of those though. E.g. I'm thinking the
> below.
> 
> static inline int __get_file_write_access(struct inode *inode,
> 					  struct vfsmount *mnt)
> {
> [...]
> 	if (!special_file(inode->i_mode)) {
> 		/*
> 		 * Balanced in __fput()
> 		 */
> 		error = mnt_want_write(mnt);
> 		if (error)
> 			put_write_access(inode);
> 	}
> 	return error;
> }

In practice I don't think this is an issue.  We were never supposed to
do mnt_want_write(mnt) for any 'struct file' that was a special_file(),
specifically because of what you mention.

Nick's use of mnt_want_write_file() was a 1:1 drop-in for
mnt_want_write().  So, if all is well in the world, there should not be
any call sites where mnt_want_write_file() gets called on a
special_file().

Future users of mnt_want_file_write() may not notice this fact, though.
This is probably worth at least a note in the documentation or perhaps a
WARN_ON().

-- Dave

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