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Date: Sun, 27 Mar 2016 22:48:10 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sedat Dilek <sedat.dilek@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [Linux-v4.6-rc1] ext4: WARNING: CPU: 2 PID: 2692 at
 kernel/locking/lockdep.c:2017 __lock_acquire+0x180e/0x2260
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On Sun, Mar 27, 2016 at 05:03:44AM -0700, Linus Torvalds wrote:
> Anyway, I don't think that DEBUG_LOCKS_WARN_ON() in
> 
>   kernel/locking/lockdep.c:2017 __lock_acquire
> 
> would be an ext4 issue, it looks more like an internal lockdep issue.
> 
> Adding in the lockdep people, who will set me right.

You are right; this is lockdep running into a hash collision; which is a
new DEBUG_LOCKDEP test. See 9e4e7554e755 ("locking/lockdep: Detect
chain_key collisions").