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Date: Wed, 14 Sep 2011 05:40:13 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Vladislav Bolkhovitin <vst@vlnb.net>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: Lockdep and rw_semaphores
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On Tue, Sep 13, 2011 at 09:55:25PM -0400, Vladislav Bolkhovitin wrote:

> > thread 1:
> > 	down_read(&A); /* got it */
> > thread 2:
> > 	down_read(&B); /* got it */
> > thread 3:
> > 	down_write(&A);	/* blocked until thread 1 releases A */

That's the only thread here doing down_write() on A

> > thread 4:
> > 	down_write(&B);	/* blocked until thread 2 releases B */

... and that's the only thread here doing down_write() on B.  And neither
of those is holding any other locks.  No nesting.

> 1. Reverse read locking isn't always a deadlock. For instance, if only 1 write
> thread participating and doesn't do nested write locking, which is a quite valid
> scenario, because by design of rw locks they are used with many readers and
> limited amount of rare writers.

Um?  If you mean that here we have two threads doing down_write(), remember
that you've got two locks.

> So, it should be better if this warning is issued, if there is >1 thread write
> locking detected on any participated rw lock, and illustrated with a correct
> explanation.

Which would be which threads, in the situation described above?  Again,
we have no nesting for writes and we have one thread attempting down_write()
for any given lock.  Two locks, two writers in total...