smp_shutdown_nonboot_cpus() repeats the same code chunk as migrate_to_reboot_cpu() to ensure that the rebooting happens on a valid cpu.
if (!cpu_online(primary_cpu)) primary_cpu = cpumask_first(cpu_online_mask); This is due to an unexpected cpu-down event like the following: kernel_kexec() migrate_to_reboot_cpu(); cpu_hotplug_enable(); -----------> comes a cpu_down(this_cpu) on other cpu machine_shutdown(); smp_shutdown_nonboot_cpus(); which needs to re-check "if (!cpu_online(primary_cpu))" Although the kexec-reboot task can get through a cpu_down() on its cpu, this code looks a little confusing. Tracing down the git history, the cpu_hotplug_enable() called by kernel_kexec() is introduced by commit 011e4b02f1da ("powerpc, kexec: Fix "Processor X is stuck" issue during kexec from ST mode"), which wakes up all offline cpu by cpu_up(cpu). Later, it is required by the architectures(arm/arm64/ia64/riscv) which resort to cpu hot-removing to achieve kexec-reboot by smp_shutdown_nonboot_cpus()->cpu_down_maps_locked(). Hence, the cpu_hotplug_enable() in kernel_kexec() is an architecture requirement. By deferring the cpu hotplug enable to a more proper point, where smp_shutdown_nonboot_cpus() holds cpu_add_remove_lock, the unexpected cpu-down event is squashed out and the rebooting cpu can keep unchanged. (For powerpc, no gains from this change.) As a result, the repeated code chunk can be removed and in [2/2], the callsites of smp_shutdown_nonboot_cpus() can be consistent. Signed-off-by: Pingfan Liu <kernelf...@gmail.com> Cc: Eric Biederman <ebied...@xmission.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Vincent Donnefort <vincent.donnef...@arm.com> Cc: Ingo Molnar <mi...@kernel.org> Cc: Michael Ellerman <m...@ellerman.id.au> Cc: Mark Rutland <mark.rutl...@arm.com> Cc: YueHaibing <yuehaib...@huawei.com> Cc: Baokun Li <libaok...@huawei.com> Cc: Randy Dunlap <rdun...@infradead.org> Cc: Valentin Schneider <valentin.schnei...@arm.com> Cc: ke...@lists.infradead.org To: linuxppc-dev@lists.ozlabs.org To: linux-ker...@vger.kernel.org --- arch/powerpc/kexec/core_64.c | 1 + kernel/cpu.c | 10 +++++----- kernel/kexec_core.c | 11 +++++------ 3 files changed, 11 insertions(+), 11 deletions(-) diff --git a/arch/powerpc/kexec/core_64.c b/arch/powerpc/kexec/core_64.c index 6cc7793b8420..8ccf22197f08 100644 --- a/arch/powerpc/kexec/core_64.c +++ b/arch/powerpc/kexec/core_64.c @@ -224,6 +224,7 @@ static void wake_offline_cpus(void) static void kexec_prepare_cpus(void) { + cpu_hotplug_enable(); wake_offline_cpus(); smp_call_function(kexec_smp_down, NULL, /* wait */0); local_irq_disable(); diff --git a/kernel/cpu.c b/kernel/cpu.c index d0a9aa0b42e8..4415370f0e91 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -1236,12 +1236,12 @@ void smp_shutdown_nonboot_cpus(unsigned int primary_cpu) 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. + * At this point, the cpu hotplug is still disabled by + * migrate_to_reboot_cpu() to guarantee that the rebooting happens on + * the selected CPU. But cpu_down_maps_locked() returns -EBUSY, if + * cpu_hotplug_disabled. So re-enable CPU hotplug 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..1bd5a8c95a20 100644 --- a/kernel/kexec_core.c +++ b/kernel/kexec_core.c @@ -1168,14 +1168,13 @@ 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 and pin the + * rebooting thread on the selected CPU. If an architecture + * requires CPU hotplug to achieve kexec reboot, it should + * enable the hotplug in the architecture specific code */ - cpu_hotplug_enable(); + pr_notice("Starting new kernel\n"); machine_shutdown(); } -- 2.31.1