linux-kernel - Re: [PATCH] smp,cpumask: Don't call functions on offline CPUs

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Message-ID: <20190522144918.GH16275@worktop.programming.kicks-ass.net>
Date:   Wed, 22 May 2019 16:49:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Rik van Riel <riel@...riel.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp,cpumask: Don't call functions on offline CPUs

On Wed, May 22, 2019 at 03:37:11PM +0100, Andrew Murray wrote:
> > Is perhaps the problem that on_each_cpu_cond() uses cpu_onlne_mask
> > without protection?
> 
> Does this prevent racing with a CPU going offline? I guess this prevents
> the warning at the expense of a lock - but is only beneficial in the
> unlikely path. (In the likely path this prevents new CPUs going offline
> but we don't care because we don't WARN if they aren't they when we
> attempt to call functions).
> 
> At least this is my limited understanding.

Hmm.. I don't think it could matter, we only use the mask when
preempt_disable(), which would already block offline, due to it using
stop_machine().

So the patch is a no-op.

What's the WARN you see? TLB invalidation should pass mm_cpumask(),
which similarly should not contain offline CPUs I'm thinking.