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

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Date:	Wed, 20 Jan 2016 22:28:07 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408!

On Wed 20-01-16 09:55:22, Christoph Lameter wrote:
[...]
> 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.
> 
> Concurrent actions could be onlining and offlining of processors or be a
> result of concurrency issues when updating the cpumask from multiple
> processors.

Now that 7e988032 ("vmstat: make vmstat_updater deferrable again and
shut down on idle) is merged the VM_BUG_ON is simply bogus because
vmstat_update might "race" with quiet_vmstat. The changelog should
reflect that. What about the following wording?

"
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")
CC: stable # 4.4+
"

> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> 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