Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408!

[Michal Hocko]

On Fri 22-01-16 10:46:14, Christoph Lameter wrote:
> On Fri, 22 Jan 2016, Michal Hocko wrote:
> 
> > Could you repost the patch with the updated description?
> 
> Subject: vmstat: Remove BUG_ON from vmstat_update
> 
> If we detect that there is nothing to do just set the flag and do not check
> if it was already set before. Races really do not matter. If the flag is
> set by any code then the shepherd will start dealing with the situation
> and reenable the vmstat workers when necessary again.
> 
> Since 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and
> shut down on idle") quiet_vmstat might update cpu_stat_off and mark a
> particular cpu to be handled by vmstat_shepherd. This might trigger
> a VM_BUG_ON in vmstat_update because the work item might have been
> sleeping during the idle period and see the cpu_stat_off updated after
> the wake up. The VM_BUG_ON is therefore misleading and no more
> appropriate. Moreover it doesn't really suite any protection from real
> bugs because vmstat_shepherd will simply reschedule the vmstat_work
> anytime it sees a particular cpu set or vmstat_update would do the same
> from the worker context directly. Even when the two would race the
> result wouldn't be incorrect as the counters update is fully idempotent.
>

Fixes: 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and shut down on idle"
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>

Would be appropriate IMO

> Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> 
> Index: linux/mm/vmstat.c
> ===================================================================
> --- linux.orig/mm/vmstat.c
> +++ linux/mm/vmstat.c
> @@ -1408,17 +1408,7 @@ static void vmstat_update(struct work_st
>  		 * Defer the checking for differentials to the
>  		 * shepherd thread on a different processor.
>  		 */
> -		int r;
> -		/*
> -		 * Shepherd work thread does not race since it never
> -		 * changes the bit if its zero but the cpu
> -		 * online / off line code may race if
> -		 * worker threads are still allowed during
> -		 * shutdown / startup.
> -		 */
> -		r = cpumask_test_and_set_cpu(smp_processor_id(),
> -			cpu_stat_off);
> -		VM_BUG_ON(r);
> +		cpumask_set_cpu(smp_processor_id(), cpu_stat_off);
>  	}
>  }

-- 
Michal Hocko
SUSE Labs