On Sun, Dec 09, 2007 at 08:46:47AM +0100, Ingo Molnar wrote:Hi Ingo, From the code I fail to see how get_online_cpus() can help us. + /* + * Only do the hung-tasks check on one CPU: + */ + get_online_cpus(); + check_cpu = any_online_cpu(cpu_online_map); + put_online_cpus(); check_cpu can go offline here, no? + + if (this_cpu != check_cpu) + continue; + + if (sysctl_hung_task_timeout_secs) + check_hung_uninterruptible_tasks(this_cpu); Further more this can cause a deadlock since we're calling get_online_cpus() from the watchdog thread's context, which is going to be kthread_stop'ed from a cpu-hotplug context. This is what I think was happening in the case reported by Jiri. Please find the patch below. Thanks and Regards gautham. commit 15bfb662b35c609490185fba2fd4713d230b9374 Author: Gautham R Shenoy <ego@in.ibm.com> Date: Mon Dec 10 13:41:45 2007 +0530 softlockup: remove get_online_cpus() which doesn't help here. The get_online_cpus() protection seems to be bogus in kernel/softlockup.c as cpu cached in check_cpu can go offline once we do a put_online_cpus(). This can also cause deadlock during a cpu offline as follows: WATCHDOG_THREAD: OFFLINE_CPU: mutex_down(&cpu_hotplug.lock); /* All subsequent get_online_cpus * will be blocked till we're * done with this cpu-hotplug * operation. */ get_online_cpus(); /* watchdog is blocked Thus we cannot go further until the cpu-hotplug operation completes */ CPU_DEAD: kthread_stop(watchdog_thread); /* we're trying to stop a * thread which is blocked * waiting for us to finish. * * Since we cannot finish until * the thread stops, we deadlock here! */ Signed-off-by: Gautham R Shenoy <ego@in.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linuxtronix.de> Cc: Arjan van de Van <arjan@linux.intel.com> Cc: Jiri Slaby <jirislaby@gmail.com> diff --git a/kernel/softlockup.c b/kernel/softlockup.c index e50b44a..576eb9c 100644 --- a/kernel/softlockup.c +++ b/kernel/softlockup.c @@ -219,9 +219,7 @@ static int watchdog(void *__bind_cpu) /* * Only do the hung-tasks check on one CPU: */ - get_online_cpus(); check_cpu = any_online_cpu(cpu_online_map); - put_online_cpus(); if (this_cpu != check_cpu) continue; -- Gautham R Shenoy Linux Technology Center IBM India. "Freedom comes with a price tag of responsibility, which is still a bargain, because Freedom is priceless!" --
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