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Date: Tue, 1 Oct 2013 23:04:53 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: spinlock contention of files->file_lock
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On Tue, Oct 01, 2013 at 02:41:58PM -0700, Eric Dumazet wrote:
> Maybe I am missing something obvious ?

Yes.  do_execve_common() starts with unshare_files(); there can be
no other thread capable of modifying that descriptor table.