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Date: Tue, 18 Sep 2007 19:38:52 +0100 (BST)
From: Hugh Dickins <hugh@veritas.com>
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To: "J. Bruce Fields" <bfields@fieldses.org>
cc: Trond Myklebust <trond.myklebust@fys.uio.no>,
       Pavel Emelyanov <xemul@openvz.org>, Andrew Morton <akpm@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       devel@openvz.org
Subject: Re: [PATCH] Wake up mandatory locks waiter on chmod (v2)
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On Tue, 18 Sep 2007, J. Bruce Fields wrote:
> On Tue, Sep 18, 2007 at 12:54:56PM -0400, Trond Myklebust wrote:
> > 
> > It gets even better when you throw mmap() into the mix :-)
> 
> Hm.  Documentation/mandatory.txt claims that it mandatory locks and
> mmap() with MAP_SHARED exclude each other, but I can't see where that's
> enfoced.  That file doesn't make any mention of the above race.

I believe the locks_verify_locked() call from mm/mmap.c prevents
mmap'ing shared-write a file with mandatory locks in force;
and the mapping_writably_mapped() calls from fs/locks.c prevent
mandatory locking on a file while it's mmap'ed shared-write.

Though I think there's no lock to prevent those checks racing,
so it's not quite watertight.

Hugh