From 7c5c92ed56d932b2c19c3f8aea86369509407d33 Mon Sep 17 00:00:00 2001 From: Anton Blanchard Date: Tue, 9 Dec 2014 10:58:19 +1100 Subject: [PATCH] powerpc: Secondary CPUs must set cpu_callin_map after setting active and online I have a busy ppc64le KVM box where guests sometimes hit the infamous "kernel BUG at kernel/smpboot.c:134!" issue during boot: BUG_ON(td->cpu != smp_processor_id()); Basically a per CPU hotplug thread scheduled on the wrong CPU. The oops output confirms it: CPU: 0 Comm: watchdog/130 The problem is that we aren't ensuring the CPU active and online bits are set before allowing the master to continue on. The master unparks the secondary CPUs kthreads and the scheduler looks for a CPU to run on. It calls select_task_rq and realises the suggested CPU is not in the cpus_allowed mask. It then ends up in select_fallback_rq, and since the active and online bits aren't set we choose some other CPU to run on. Cc: stable@vger.kernel.org Signed-off-by: Anton Blanchard Signed-off-by: Michael Ellerman --- arch/powerpc/kernel/smp.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index 8b2d2dc8ef10..8ec017cb4446 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -700,7 +700,6 @@ void start_secondary(void *unused) smp_store_cpu_info(cpu); set_dec(tb_ticks_per_jiffy); preempt_disable(); - cpu_callin_map[cpu] = 1; if (smp_ops->setup_cpu) smp_ops->setup_cpu(cpu); @@ -739,6 +738,14 @@ void start_secondary(void *unused) notify_cpu_starting(cpu); set_cpu_online(cpu, true); + /* + * CPU must be marked active and online before we signal back to the + * master, because the scheduler needs to see the cpu_online and + * cpu_active bits set. + */ + smp_wmb(); + cpu_callin_map[cpu] = 1; + local_irq_enable(); cpu_startup_entry(CPUHP_ONLINE); -- 2.20.1