linux-kernel - Re: [PATCH] sched: fix cpu_active_mask/cpu_online

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Date:	Thu, 30 Jul 2015 18:00:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan H. Schönherr <jschoenh@...zon.de>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Joerg Roedel <jroedel@...e.de>, Borislav Petkov <bp@...en8.de>,
	linux-kernel@...r.kernel.org, Lai Jiangshan <laijs@...fujitsu.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Anton Blanchard <anton@...ba.org>, Matt Wilson <msw@...zon.com>
Subject: Re: [PATCH] sched: fix cpu_active_mask/cpu_online_mask race

On Mon, Jul 27, 2015 at 08:21:28PM +0200, Jan H. Schönherr wrote:
> From: Jan H. Schönherr <jschoenh@...zon.de>
> 
> Together, these give us the following non-working solutions:
> - secondary CPU sets active before online, because active is assumed to
>   be a subset of online;
> - secondary CPU sets online before active, because the primary CPU
>   assumes that an online CPU is also active;
> - secondary CPU sets online and waits for primary CPU to set active,
>   because it might deadlock.
> 
> Commit 875ebe940d77 ("powerpc/smp: Wait until secondaries are active &
> online") introduces an arch-specific solution to this arch-independent
> problem.
> 
> Now, go for a more general solution without explicit waiting and simply
> set active twice: once on the secondary CPU after online was set and
> once on the primary CPU after online was seen.

Very vile indeed ;-)

> Fixes: 6acbfb96976f ("sched: Fix hotplug vs. set_cpus_allowed_ptr()")
> Signed-off-by: Jan H. Schönherr <jschoenh@...zon.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  kernel/sched/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 78b4bad10..155bb4a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5433,6 +5433,7 @@ static int sched_cpu_active(struct notifier_block *nfb,
>  	case CPU_STARTING:
>  		set_cpu_rq_start_time();
>  		return NOTIFY_OK;
> +	case CPU_ONLINE:

Not without a big honking comment right there :-)

>  	case CPU_DOWN_FAILED:
>  		set_cpu_active((long)hcpu, true);
>  		return NOTIFY_OK;
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