Date: Wed, 4 Nov 2009 08:24:18 -0600
From: "Serge E. Hallyn" <serue@us.ibm.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>, Kay Sievers <kay.sievers@vrfy.org>,
       Greg KH <greg@kroah.com>, linux-kernel@vger.kernel.org,
       Tejun Heo <tj@kernel.org>, Cornelia Huck <cornelia.huck@de.ibm.com>,
       linux-fsdevel@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>,
       Benjamin LaHaise <bcrl@lhnet.ca>,
       "Eric W. Biederman" <ebiederm@aristanetworks.com>
Subject: Re: [PATCH 09/13] sysfs: Implement sysfs_getattr & sysfs_permission
Message-ID: <20091104142418.GB4355@us.ibm.com>
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Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serue@us.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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