Xiaotian Feng wrote:
Could follow be possible? We know there's cpu 0 and cpu 1,offline cpu1 > done offline cpu0 > false consider this in cpu_down code, int __ref cpu_down(unsigned int cpu) { <snip> set_cpu_active(cpu, false); // here, we set cpu 0 to inactive synchronize_sched(); err = _cpu_down(cpu, 0); out: <snip> } Then in _cpu_down code: static int __ref _cpu_down(unsigned int cpu, int tasks_frozen) { <snip> if (num_online_cpus() == 1) // if we're trying to offline cpu0, num_online_cpus will be 1 return -EBUSY; // after return back to cpu_down, we didn't change cpu 0 back to active if (!cpu_online(cpu)) return -EINVAL; if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL)) return -ENOMEM; <snip> } Then cpu 0 is not active, but online, then we try to offline cpu1, ....... This can not be exposed because x86 does not have /sys/devices/system/cpu0/online. I guess following patch fixes this bug.
Just tested this one on the POWER box and the test passed. I did not observe the hang. Thanks -Sachin
--- diff --git a/kernel/cpu.c b/kernel/cpu.c index 291ac58..21ddace 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -199,14 +199,18 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen) .hcpu = hcpu, }; - if (num_online_cpus() == 1) + if (num_online_cpus() == 1) { + set_cpu_active(cpu, true); return -EBUSY; + } if (!cpu_online(cpu)) return -EINVAL; - if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL)) + if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL)) { + set_cpu_active(cpu, true); return -ENOMEM; + } cpu_hotplug_begin(); err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,Unless of course, I messed up, which appears to be rather likely given these problems ;-)
-- --------------------------------- Sachin Sant IBM Linux Technology Center India Systems and Technology Labs Bangalore, India --------------------------------- _______________________________________________ Linuxppc-dev mailing list [email protected] https://lists.ozlabs.org/listinfo/linuxppc-dev
