Re: broken suspend (sched related) [Was: 2.6.24-rc4-mm1]

[Jiri Slaby <jirislaby@gmail.com>]

On 12/10/2007 09:19 AM, Gautham R Shenoy wrote:
> 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>

Tested-by: 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;
> 
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