Re: [PATCH 09/13] sysfs: Implement sysfs_getattr & sysfs_permission

From: Serge E. Hallyn
Date: Wed Nov 04 2009 - 09:24:34 EST

Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
> "Serge E. Hallyn" <serue@xxxxxxxxxx> 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...

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