linux-kernel - Re: [RFC][PATCH 1/7] sched: Fix smp_call_function_single

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Message-ID: <jhjr1v2iwz7.mognet@arm.com>
Date:   Fri, 29 May 2020 16:26:04 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, frederic@...nel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org, cai@....pw,
        mgorman@...hsingularity.net
Subject: Re: [RFC][PATCH 1/7] sched: Fix smp_call_function_single_async() usage for ILB


On 26/05/20 17:10, Peter Zijlstra wrote:
> The recent commit: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
> got smp_call_function_single_async() subtly wrong. Even though it will
> return -EBUSY when trying to re-use a csd, that condition is not
> atomic and still requires external serialization.
>
> The change in kick_ilb() got this wrong.
>
> While on first reading kick_ilb() has an atomic test-and-set that
> appears to serialize the use, the matching 'release' is not in the
> right place to actually guarantee this serialization.
>

I got confused the first time I read through that, and had the same
thoughts as Vincent; reading Frederic's reply and the thread
helped. Could we mention the tick softirq vs ilb kick race
thing in the changelog?

Otherwise the change looks okay, I couldn't convince myself there were
more gaps left to fill.

Reviewed-by: Valentin Schneider <valentin.schneider@....com>

> Rework the nohz_idle_balance() trigger so that the release is in the
> IPI callback and thus guarantees the required serialization for the
> CSD.
>
> Fixes: 90b5363acd47 ("sched: Clean up scheduler_ipi()")
> Reported-by: Qian Cai <cai@....pw>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>