linux-kernel - Re: [PATCH] workqueue: Protects wq_unbound_cpumask with wq_pool_attach

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Message-ID: <YxUJPrRGmUTQu5VS@slm.duckdns.org>
Date:   Sun, 4 Sep 2022 10:23:26 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Zqiang <qiang1.zhang@...el.com>
Subject: Re: [PATCH] workqueue: Protects wq_unbound_cpumask with
 wq_pool_attach_mutex

Hello,

On Tue, Aug 30, 2022 at 05:32:17PM +0800, Lai Jiangshan wrote:
> > Is this enough? Shouldn't the lock be protecting a wider scope? If there's
> > someone reading the flag with just pool_attach_mutex, what prevents them
> > reading it right before the new value is committed and keeps using the stale
> > value?
> 
> Which "flag"? wq_unbound_cpumask?

Oh, yeah, sorry.

> This code is adding protection for wq_unbound_cpumask and makes
> unbind_workers() use a stable version of wq_unbound_cpumask during
> operation.
> 
> It doesn't really matter if pool's mask becomes stale later again
> with respect to wq_unbound_cpumask.
> 
> No code ensures the disassociated pool's mask is kept with the newest
> wq_unbound_cpumask since the 10a5a651e3af ("workqueue: Restrict kworker
> in the offline CPU pool running on housekeeping CPUs") first uses
> wq_unbound_cpumask for the disassociated pools.
> 
> What matters is that the pool's mask should the wq_unbound_cpumask
> at the time when it becomes disassociated which has no isolated CPUs.
> 
> I don't like 10a5a651e3af for it not synching the pool's mask
> with wq_unbound_cpumask. But I think it works anyway.

Hmm... I see. Can you add a comment explaining why we're grasbbing
wq_pool_attach_mutex there?

Thanks.

-- 
tejun