linux-kernel - Re: [PATCH 09/13] sysfs: Implement sysfs_getattr & sysfs

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Date:	Wed, 4 Nov 2009 08:24:18 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Kay Sievers <kay.sievers@...y.org>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-fsdevel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Benjamin LaHaise <bcrl@...et.ca>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 09/13] sysfs: Implement sysfs_getattr & sysfs_permission

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serue@...ibm.com> writes:
> 
> > So the inode->i_mutex is not needed?
> 
> Good question.  Nothing in sysfs needs it.  The VFS does not grab the
> inode mutex on this path, but the vfs does grab the inode mutex when
> writing to the inode.

All callers of fs/attr.c:notify_change() do seem to take the i_mutex,
though.  And Documentation/filesystem/Locking claims that ->setattr()
does need i_mutex.  So I assume that setting of inode->i_ctime etc,
which is what you're doing here, needs to be protected by the i_mutex.

> Since the VFs isn't grabbing the inode_mutex there is probably a race in
> here somewhere if someone looks at things just right.
> 
> I am too tired tonight to be that person.

The readers take no lock of any sort (i.e. generic_fillattr and its
callers) so IIUC they could get inconsistent data...

-serge
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