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Date: Fri, 27 Apr 2007 22:48:44 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hugh Dickins <hugh@veritas.com>
Cc: Matt Mackall <mpm@selenic.com>, Alexey Dobriyan <adobriyan@sw.ru>,
       linux-kernel@vger.kernel.org, Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: - maps2-add-proc-pid-pagemap-interface-fix.patch removed from
 -mm tree
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On Sat, 28 Apr 2007 06:13:39 +0100 (BST) Hugh Dickins <hugh@veritas.com> wrote:

> On Fri, 27 Apr 2007, Andrew Morton wrote:
> > 
> > hm, could do.  might_sleep() is intertwined with preempt in complex ways,
> > but we did decouple that at the config level.  no_mmap_sem() will dtrt for
> > all preempt settings.
> > 
> > But I'll be keeping this as a -mm-only debug patch (which brings us up to
> > about thirty of 'em), so I think it's best to make it unconfigurable so we
> > get maximum coverage.
> > 
> > That's if it actually works.  I haven't tried running it yet, and I have a
> > feeling that running it might cause a big "doh" moment.  We'll see.
> 
> Yes, I'm expecting the crucial
> 
> > +	WARN_ON(rwsem_is_locked(&mm->mmap_sem))
> 
> to give a bogus warning every time another thread (or /proc,
> or swapoff, or whatever) happens to have this mmap_sem locked.
> might_sleep() is quite different, works on our thread's info.
> 

Yes.  lockdep has a way of working out if this task already has a
particular lock for reading or writing, but it isn't immediately obvious
how to extract that.

I guess a simple hack would be do do a down_read() on it.  If it's already
held for reading, lockdep should warn.  If it's already held for writing
someone will notice.

Oh well, it's not my top priority.