linux-kernel - Re: [RESEND PATCH 1/3] completion: Add support for initializing completion with lockdep

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Message-ID: <1508529532.3029.15.camel@wdc.com>
Date:   Fri, 20 Oct 2017 19:58:54 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "amir73il@...il.com" <amir73il@...il.com>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
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        "byungchul.park@....com" <byungchul.park@....com>,
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        "kernel-team@....com" <kernel-team@....com>,
        "david@...morbit.com" <david@...morbit.com>
Subject: Re: [RESEND PATCH 1/3] completion: Add support for initializing
 completion with lockdep_map

On Fri, 2017-10-20 at 08:34 +0200, Thomas Gleixner wrote:
> On Thu, 19 Oct 2017, Bart Van Assche wrote:
> > Are there any completion objects for which the cross-release checking is
> > useful?
> 
> All of them by definition.

Sorry but I'm not sure that's the best possible answer. In my opinion
avoiding that completion objects have dependencies on other lock objects,
e.g. by avoiding to wait on a completion object while holding a mutex, is a
far superior strategy over adding cross-release checking to completion
objects. The former strategy namely makes it unnecessary to add
cross-release checking to completion objects because that strategy ensures
that these completion objects cannot get involved in a deadlock. The latter
strategy can lead to false positive deadlock reports by the lockdep code,
something none of us wants.

A possible alternative strategy could be to enable cross-release checking
only for those completion objects for which waiting occurs inside a critical
section.

> > Are there any wait_for_completion() callers that hold a mutex or
> > other locking object?
> 
> Yes, there are also cross completion dependencies. There have been such
> bugs and I expect more to be unearthed.
> 
> I really have to ask what your motiviation is to fight the lockdep coverage
> of synchronization objects tooth and nail?

As explained in another e-mail thread, unlike the lock inversion checking
performed by the <= v4.13 lockdep code, cross-release checking is a heuristic
that does not have a sound theoretical basis. The lock validator is an
important tool for kernel developers. It is important that it produces as few
false positives as possible. Since the cross-release checks are enabled
automatically when enabling lockdep, I think it is normal that I, as a kernel
developer, care that the cross-release checks produce as few false positives
as possible.

Bart.