linux-kernel - Re: [PATCH 1/3] sysfs: Remove lock ordering violation in sysfs_chmod

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Date:	Tue, 07 Oct 2008 15:31:21 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Greg KH <greg@...ah.com>, Al Viro <viro@...IV.linux.org.uk>,
	Benjamin Thery <benjamin.thery@...l.net>,
	linux-kernel@...r.kernel.org, "Serge E. Hallyn" <serue@...ibm.com>,
	Al Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/3] sysfs:  Remove lock ordering violation in sysfs_chmod_file.

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

> On Tue, 2008-10-07 at 03:49 -0700, Eric W. Biederman wrote:
>> It is a wee bit subtle but sysfs_get_dentry grabs inode->i_mutex.
>> of potentially all of the parents of sd.  So I can not hold
>> the inode mutex of the directory while it is called.
>
> Hi Eric,
>
> This patch looks good to me.  In my quest to parse exactly what was
> going on, I rewrote a description of the patch.  Could you look over
> this and see if I'm on target?

It sounds like you are on target.

> Before this patch, inode->i_mutex is held in order to keep the inode's
> mode and ctime from changing out underneath us.  If we didn't do this,
> you could potentially get garbage when reading them out of the old
> inode.  We calculated these new permissions once since it is redundant
> to do it several times.

Yes.  updates to mtime and ctime need to be serialized by holding
the inode semaphore.

> We also need to perform a sysfs_get_dentry() operation on the
> sysfs_dirent in order to find all the dentries on each sb.

To find the dentry for a particular sb.

> This needs
> to be performed once for each sb in which the inode appears.  "[B]ut
> sysfs_get_dentry grabs inode->i_mutex. of potentially all of the parents
> of sd.  So I can not hold the inode mutex of the directory while it is
> called."

Yes.

> This patch drops the inode->i_mutex over the entire "for each
> sysfs_dirent" loop.  It, instead, reacquires and drops it each time we
> need to calculate the new mode/ctime for the target dentries.  It does
> this away from the sysfs_get_dentry() call now.

Yes.

Eric
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