linux-kernel - Re: [locking/ww_mutex] 2a0c112828 WARNING: CPU: 0 PID: 18 at kernel/locking/mutex.c:305 __ww_mutex_wakeup_for

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Date:   Thu, 2 Mar 2017 01:39:16 +0800
From:   Fengguang Wu <fengguang.wu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        LKP <lkp@...org>
Subject: Re: [locking/ww_mutex] 2a0c112828 WARNING: CPU: 0 PID: 18 at
 kernel/locking/mutex.c:305 __ww_mutex_wakeup_for_backoff

On Wed, Mar 01, 2017 at 05:54:06PM +0100, Peter Zijlstra wrote:
>On Wed, Mar 01, 2017 at 04:26:48PM +0000, Chris Wilson wrote:
>> On Wed, Mar 01, 2017 at 04:11:48PM +0000, Chris Wilson wrote:
>> > On Wed, Mar 01, 2017 at 04:54:14PM +0100, Peter Zijlstra wrote:
>> > > On Wed, Mar 01, 2017 at 11:40:43PM +0800, Fengguang Wu wrote:
>> > > > Thanks for the patch! I applied the patch on top of "locking/ww_mutex:
>> > > > Add kselftests for ww_mutex stress", and find no "bad unlock balance
>> > > > detected" but this warning. Attached is the new dmesg which is a bit
>> > > > large due to lots of repeated errors.
>> > >
>> > > So with all the various patches it works for me.
>> > >
>> > > I also have the following on top; which I did when I was looking through
>> > > this code trying to figure out wth was happening.
>> > >
>> > > Chris, does this make sense to you?
>> > >
>> > > It makes each loop a fully new 'instance', otherwise we'll never update
>> > > the ww_class->stamp and the threads will aways have the same order.
>> >
>> > Sounds ok, I just thought the stamp order of the threads was
>> > immaterial - with each test doing a different sequence of locks and each
>> > being identical in behaviour, it would not matter which had priority,
>> > there would have be some shuffling no matter waht. However, for the
>> > purpose of testing, having each iteration be a new locking instance does
>> > make it behaviour more like a typical user.
>>
>> Correcting myself, the workers didn't reorder the locks, so changing the
>> stamp does make the test more interesting.
>
>OK, so I'll go write a Changelog for it then ;-) And stick your ACK on.

With both patches in this thread, all 110 boots are successful w/o a
single warning.

Tested-by: Fengguang Wu <fengguang.wu@...el.com>

Thanks,
Fengguang