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

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Date:   Tue, 30 Aug 2022 17:32:17 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Tejun Heo <tj@...nel.org>
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

On Sat, Aug 27, 2022 at 8:33 AM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Thu, Aug 18, 2022 at 10:33:48PM +0800, Lai Jiangshan wrote:
> > @@ -5342,6 +5344,11 @@ static int workqueue_apply_unbound_cpumask(void)
> >               apply_wqattrs_cleanup(ctx);
> >       }
> >
> > +     if (!ret) {
> > +             mutex_lock(&wq_pool_attach_mutex);
> > +             cpumask_copy(wq_unbound_cpumask, unbound_cpumask);
> > +             mutex_unlock(&wq_pool_attach_mutex);
>
> 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?

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.

>
> Thanks.
>
> --
> tejun