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Date: Thu, 27 Jan 2022 17:41:44 +0800 From: Baoquan He <bhe@...hat.com> To: Pingfan Liu <kernelfans@...il.com> Cc: linux-kernel@...r.kernel.org, Eric Biederman <ebiederm@...ssion.com>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Valentin Schneider <valentin.schneider@....com>, Vincent Donnefort <vincent.donnefort@....com>, Ingo Molnar <mingo@...nel.org>, Mark Rutland <mark.rutland@....com>, YueHaibing <yuehaibing@...wei.com>, Baokun Li <libaokun1@...wei.com>, Randy Dunlap <rdunlap@...radead.org>, kexec@...ts.infradead.org Subject: Re: [PATCHv2] kexec: disable cpu hotplug until the rebooting cpu is stable Hi Pingfan, On 01/27/22 at 05:02pm, Pingfan Liu wrote: > The following identical code piece appears in both > migrate_to_reboot_cpu() and smp_shutdown_nonboot_cpus(): > > if (!cpu_online(primary_cpu)) > primary_cpu = cpumask_first(cpu_online_mask); > > This is due to a breakage like the following: > migrate_to_reboot_cpu(); > cpu_hotplug_enable(); > --> comes a cpu_down(this_cpu) on other cpu > machine_shutdown(); > > Although the kexec-reboot task can get through a cpu_down() on its cpu, > this code looks a little confusing. > > Make things straight forward by keeping cpu hotplug disabled until > smp_shutdown_nonboot_cpus() holds cpu_add_remove_lock. By this way, the > breakage is squashed out and the rebooting cpu can keep unchanged. If I didn't go through code wrongly, you may miss the x86 case. Several ARCHes do call smp_shutdown_nonboot_cpus() in machine_shutdown() in kexec reboot code path, while x86 doesn't. If I am right, you may need reconsider if this patch is needed or need be adjustd. Are you optimizing code path, or you meet a real problem? I haven't checked v1, but I also didn't see it's told in patch log which case it is. > > Note: this patch only affects the kexec-reboot on arches, which rely on > cpu hotplug mechanism. > > Signed-off-by: Pingfan Liu <kernelfans@...il.com> > Cc: Eric Biederman <ebiederm@...ssion.com> > Cc: Peter Zijlstra <peterz@...radead.org> > Cc: Thomas Gleixner <tglx@...utronix.de> > Cc: Valentin Schneider <valentin.schneider@....com> > Cc: Vincent Donnefort <vincent.donnefort@....com> > Cc: Ingo Molnar <mingo@...nel.org> > Cc: Mark Rutland <mark.rutland@....com> > Cc: YueHaibing <yuehaibing@...wei.com> > Cc: Baokun Li <libaokun1@...wei.com> > Cc: Randy Dunlap <rdunlap@...radead.org> > Cc: Valentin Schneider <valentin.schneider@....com> > Cc: kexec@...ts.infradead.org > To: linux-kernel@...r.kernel.org > --- > v1 -> v2: > improve commit log > > kernel/cpu.c | 16 ++++++++++------ > kernel/kexec_core.c | 10 ++++------ > 2 files changed, 14 insertions(+), 12 deletions(-) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 9c92147f0812..87bdf21de950 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -1240,20 +1240,24 @@ int remove_cpu(unsigned int cpu) > } > EXPORT_SYMBOL_GPL(remove_cpu); > > +/* primary_cpu keeps unchanged after migrate_to_reboot_cpu() */ > void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) > { > unsigned int cpu; > int error; > > + /* > + * Block other cpu hotplug event, so primary_cpu is always online if > + * it is not touched by us > + */ > cpu_maps_update_begin(); > - > /* > - * Make certain the cpu I'm about to reboot on is online. > - * > - * This is inline to what migrate_to_reboot_cpu() already do. > + * migrate_to_reboot_cpu() disables CPU hotplug assuming that > + * no further code needs to use CPU hotplug (which is true in > + * the reboot case). However, the kexec path depends on using > + * CPU hotplug again; so re-enable it here. > */ > - if (!cpu_online(primary_cpu)) > - primary_cpu = cpumask_first(cpu_online_mask); > + __cpu_hotplug_enable(); > > for_each_online_cpu(cpu) { > if (cpu == primary_cpu) > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c > index 68480f731192..db4fa6b174e3 100644 > --- a/kernel/kexec_core.c > +++ b/kernel/kexec_core.c > @@ -1168,14 +1168,12 @@ int kernel_kexec(void) > kexec_in_progress = true; > kernel_restart_prepare("kexec reboot"); > migrate_to_reboot_cpu(); > - > /* > - * migrate_to_reboot_cpu() disables CPU hotplug assuming that > - * no further code needs to use CPU hotplug (which is true in > - * the reboot case). However, the kexec path depends on using > - * CPU hotplug again; so re-enable it here. > + * migrate_to_reboot_cpu() disables CPU hotplug. If an arch > + * relies on the cpu teardown to achieve reboot, it needs to > + * re-enable CPU hotplug there. > */ > - cpu_hotplug_enable(); > + > pr_notice("Starting new kernel\n"); > machine_shutdown(); > } > -- > 2.31.1 >
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