linux-kernel - Re: mnt_want_write

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Date:	Tue, 04 Aug 2009 03:48:34 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
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?

Dave Hansen <dave@...ux.vnet.ibm.com> writes:

> 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().

void file_update_time(struct file *file)
sys_fchmod()
sys_fchown()
sys_fsetxattr()
sys_fremovexattr()

Um..., the users of mnt_want_write_file() seems to be those. I think
all of those filp can be special file?

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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