linux-kernel - Re: [PATCH v2 2/3] lockdep: Remove BROKEN flag of LOCKDEP

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Message-ID: <alpine.DEB.2.20.1710192250140.2054@nanos>
Date:   Thu, 19 Oct 2017 22:53:06 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bart Van Assche <Bart.VanAssche@....com>
cc:     "willy@...radead.org" <willy@...radead.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "byungchul.park@....com" <byungchul.park@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kernel-team@....com" <kernel-team@....com>
Subject: Re: [PATCH v2 2/3] lockdep: Remove BROKEN flag of
 LOCKDEP_CROSSRELEASE

On Thu, 19 Oct 2017, Bart Van Assche wrote:
> On Thu, 2017-10-19 at 13:33 -0700, Matthew Wilcox wrote:
> > For example, the page lock is not annotatable with lockdep -- we return
> > to userspace with it held, for heaven's sake!  So it is quite easy for
> > someone not familiar with the MM locking hierarchy to inadvertently
> > introduce an ABBA deadlock against the page lock.  (ie me.  I did that.)
> > Right now, that has to be caught by a human reviewer; if cross-release
> > checking can catch that, then it's worth having.
> 
> Hello Matthew,
> 
> Although I agree that enabling lock inversion checking for page locks is
> useful, I think my questions still apply to other locking objects than page
> locks.

Why are other objects any different?

    lock(L)   ->      wait_for_completion(A)
    lock(L)   ->      complete(A)

is a simple ABBA and they exist and have not been caught for a long time
until they choked a production machine.

Thanks,

	tglx