On 12/10/2007 09:19 AM, Gautham R Shenoy wrote:
> commit 15bfb662b35c609490185fba2fd4713d230b9374
> Author: Gautham R Shenoy <[email protected]>
> 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 <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Arjan van de Van <[email protected]>
> Cc: Jiri Slaby <[email protected]>
Tested-by: Jiri Slaby <[email protected]>
> 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;
>
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