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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Qais Yousef <qais.yousef@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Qais Yousef <qais.yousef@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Enrico Weigelt <info@metux.net>, Ram Pai <linuxram@us.ibm.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 03/12] powerpc: Replace cpu_up/down with device_online/offline
In-Reply-To: <20191030153837.18107-4-qais.yousef@arm.com>
References: <20191030153837.18107-1-qais.yousef@arm.com> <20191030153837.18107-4-qais.yousef@arm.com>
Date:   Tue, 19 Nov 2019 12:19:39 +1100
Message-ID: <87h830d5n8.fsf@mpe.ellerman.id.au>
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Sender: linux-kernel-owner@vger.kernel.org
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Qais Yousef <qais.yousef@arm.com> writes:
> The core device API performs extra housekeeping bits that are missing
> from directly calling cpu_up/down.
>
> See commit a6717c01ddc2 ("powerpc/rtas: use device model APIs and
> serialization during LPM") for an example description of what might go
> wrong.
>
> This also prepares to make cpu_up/down a private interface for anything
> but the cpu subsystem.
>
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> CC: Paul Mackerras <paulus@samba.org>
> CC: Michael Ellerman <mpe@ellerman.id.au>
> CC: Enrico Weigelt <info@metux.net>
> CC: Ram Pai <linuxram@us.ibm.com>
> CC: Nicholas Piggin <npiggin@gmail.com>
> CC: Thiago Jung Bauermann <bauerman@linux.ibm.com>
> CC: Christophe Leroy <christophe.leroy@c-s.fr>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: linuxppc-dev@lists.ozlabs.org
> CC: linux-kernel@vger.kernel.org
> ---
>  arch/powerpc/kernel/machine_kexec_64.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

My initial though is "what about kdump", but that function is not called
during kdump, so there should be no issue with the extra locking leading
to deadlocks in a crash.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

I assume you haven't actually tested it?

cheers

> diff --git a/arch/powerpc/kernel/machine_kexec_64.c b/arch/powerpc/kernel/machine_kexec_64.c
> index 04a7cba58eff..ebf8cc7acc4d 100644
> --- a/arch/powerpc/kernel/machine_kexec_64.c
> +++ b/arch/powerpc/kernel/machine_kexec_64.c
> @@ -208,13 +208,15 @@ static void wake_offline_cpus(void)
>  {
>  	int cpu = 0;
>  
> +	lock_device_hotplug();
>  	for_each_present_cpu(cpu) {
>  		if (!cpu_online(cpu)) {
>  			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",
>  			       cpu);
> -			WARN_ON(cpu_up(cpu));
> +			WARN_ON(device_online(get_cpu_device(cpu)));
>  		}
>  	}
> +	unlock_device_hotplug();
>  }
>  
>  static void kexec_prepare_cpus(void)
> -- 
> 2.17.1