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

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Date:   Wed, 27 May 2020 01:56:34 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, 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 Tue, May 26, 2020 at 06:10:58PM +0200, 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.
> 
> 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>

Looks very good! Thanks!

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>