linux-kernel - Re: Lockdep splat involving all_q

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Date:   Wed, 10 May 2017 20:55:54 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     paulmck@...ux.vnet.ibm.com, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        tglx@...utronix.de
Subject: Re: Lockdep splat involving all_q_mutex

On 05/10/2017 04:34 PM, Paul E. McKenney wrote:
> Hello!
> 
> I got the lockdep splat shown below during some rcutorture testing (which
> does CPU hotplug operations) on mainline at commit dc9edaab90de ("Merge
> tag 'acpi-extra-4.12-rc1' of git://git.kernel.org/.../rafael/linux-pm").
> My kneejerk reaction was just to reverse the "mutex_lock(&all_q_mutex);"
> and "get_online_cpus();" in blk_mq_init_allocated_queue(), but then
> I noticed that commit eabe06595d62 ("block/mq: Cure cpu hotplug lock
> inversion") just got done moving these two statements in the other
> direction.

The problem is that that patch got merged too early, as it only
fixes a lockdep splat with the cpu hotplug rework. Fix is coming Linus'
way, it's in my for-linus tree.

-- 
Jens Axboe