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Date: Wed, 2 Oct 2013 11:20:37 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Ingo Molnar <mingo@kernel.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Waiman Long <Waiman.Long@hp.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: spinlock contention of files->file_lock
Message-ID: <20131002102037.GZ13318@ZenIV.linux.org.uk>
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On Wed, Oct 02, 2013 at 07:13:19AM +0200, Ingo Molnar wrote:
> 
> * Al Viro <viro@ZenIV.linux.org.uk> wrote:
> 
> > 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.
> 
> Btw., might the Android Binder:
> 
>   drivers/staging/android/binder.c:       struct files_struct *files = proc->files;
> ...
>   drivers/staging/android/binder.c:               __fd_install(proc->files, fd, file);
> ...
>   drivers/staging/android/binder.c:       retval = __close_fd(proc->files, fd);
> 
> violate that assumption?

Not unless your thread has managed to call an ioctl between entering
do_execve_common() and calling do_close_on_exec() ;-)