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Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hdAXP-0006NA-Nx; Tue, 18 Jun 2019 09:36:23 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hdAXL-0006Ln-TW for linux-arm-kernel@lists.infradead.org; Tue, 18 Jun 2019 09:36:21 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 60AE7344; Tue, 18 Jun 2019 02:36:19 -0700 (PDT) Received: from [10.1.196.105] (eglon.cambridge.arm.com [10.1.196.105]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 633BA3F246; Tue, 18 Jun 2019 02:36:18 -0700 (PDT) Subject: Re: [PATCH] arm64: smp: fix smp_send_stop() behaviour To: Will Deacon , Cristian Marussi References: <20190613122146.45459-1-cristian.marussi@arm.com> <20190617180913.GN30800@fuggles.cambridge.arm.com> From: James Morse Message-ID: Date: Tue, 18 Jun 2019 10:36:16 +0100 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190617180913.GN30800@fuggles.cambridge.arm.com> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190618_023619_994878_0D89AD7F X-CRM114-Status: GOOD ( 18.22 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mark.rutland@arm.com, catalin.marinas@arm.com, dave.martin@arm.com, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Christian, Will, On 17/06/2019 19:09, Will Deacon wrote: > On Thu, Jun 13, 2019 at 01:21:46PM +0100, Cristian Marussi wrote: >> On a 2-CPUs system, when one CPU is already online if the other >> panics while starting-up, smp_send_stop() will fail to send any >> STOP message to the other already online core, resulting in a >> system still responsive and alive at the end of the panic procedure. >> This patch makes smp_send_stop() account also for the online status >> of the calling CPU while evaluating how many CPUs are effectively >> online: this way, an adequate number of STOPs is sent, so enforcing >> a proper freeze of the system at the end of panic even under the >> above conditions. >> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c >> index bb4b3f07761a..c7d604427883 100644 >> --- a/arch/arm64/kernel/smp.c >> +++ b/arch/arm64/kernel/smp.c >> @@ -971,8 +971,14 @@ void tick_broadcast(const struct cpumask *mask) >> void smp_send_stop(void) >> { >> unsigned long timeout; >> + unsigned int this_cpu_online = cpu_online(smp_processor_id()); >> >> - if (num_online_cpus() > 1) { >> + /* >> + * If this CPU isn't fully online, it will not be counted in >> + * num_online_cpus(): on a 2-CPU system this situation will >> + * result in no message being sent to the other already online CPU. >> + */ >> + if (num_online_cpus() > this_cpu_online) { >> cpumask_t mask; >> >> cpumask_copy(&mask, cpu_online_mask); >> @@ -985,10 +991,10 @@ void smp_send_stop(void) >> >> /* Wait up to one second for other CPUs to stop */ >> timeout = USEC_PER_SEC; >> - while (num_online_cpus() > 1 && timeout--) >> + while (num_online_cpus() > this_cpu_online && timeout--) >> udelay(1); >> >> - if (num_online_cpus() > 1) >> + if (num_online_cpus() > this_cpu_online) >> pr_warning("SMP: failed to stop secondary CPUs %*pbl\n", >> cpumask_pr_args(cpu_online_mask)); > > Whilst this looks ok to me, I'm worried about whether or not we have this > sort of logic elsewhere. For example, do we need to fix > crash_smp_send_stop() (and possibly machine_kexec()) too? We should do the same in crash_smp_send_stop(), its possible a newly-online core triggers kdump by calling panic() in the same way. machine_kexec() is called on the last surviving cpu after migrate_to_reboot_cpu() has run. At first glance it looks like this could never happen there, but kexec re-enables cpu-hotplug (commit 011e4b02f1da), and we can reschedule before we start moving memory around, so I'm not convinced its immune... Thanks, James _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel