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Date: Thu, 21 Jan 2016 17:51:49 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Christoph Lameter <cl@linux.com>
Cc: Sasha Levin <sasha.levin@oracle.com>, LKML <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408!
Message-ID: <20160121165148.GF29520@dhcp22.suse.cz>
References: <20160120143719.GF14187@dhcp22.suse.cz>
 <569FA01A.4070200@oracle.com>
 <20160120151007.GG14187@dhcp22.suse.cz>
 <alpine.DEB.2.20.1601200919520.21490@east.gentwo.org>
 <569FAC90.5030407@oracle.com>
 <alpine.DEB.2.20.1601200954420.23983@east.gentwo.org>
 <20160120212806.GA26965@dhcp22.suse.cz>
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On Thu 21-01-16 09:45:12, Christoph Lameter wrote:
> On Thu, 21 Jan 2016, Michal Hocko wrote:
[...]
> > The vmstat update might be still waiting for its timer, idle mode started
> > and kick vmstat_update which might cpumask_test_and_set_cpu. Once the
> > idle terminates and the originally schedule vmstate_update executes it
> > sees the bit set and BUG_ON.
> 
> Ok so we are going into idle mode and the vmstat_update timer is pending.
> Then the timer will not fire since going idle switches preemption off.
> quiet_vmstat will run without the chance of running vmstat_update
> 
> We could be going idle and not have disabled preemption yet. Then
> vmstat_update will run. On return to the idling operation preemption will
> be disabled and quiet_vmstat() will be run.
> 
> I do not see how these two things could race.

It goes like this:
CPU0:						CPU1
vmstat_update
  cpumask_test_and_set_cpu (0->1)
[...]
						vmstat_shepherd
<enter idle>					  cpumask_test_and_clear_cpu(CPU0) (1->0)
quiet_vmstat
  cpumask_test_and_set_cpu (0->1)
  						  queue_delayed_work_on(CPU0)
refresh_cpu_vm_stats()
[...]
vmstat_update
  nothing_to_do
  cpumask_test_and_set_cpu (1->1)
  VM_BUG_ON

Or am I missing something?
-- 
Michal Hocko
SUSE Labs

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Date: Thu, 21 Jan 2016 17:51:49 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408!
Message-ID: <20160121165148.GF29520@dhcp22.suse.cz>
References: <20160120143719.GF14187@dhcp22.suse.cz>
 <569FA01A.4070200@oracle.com>
 <20160120151007.GG14187@dhcp22.suse.cz>
 <alpine.DEB.2.20.1601200919520.21490@east.gentwo.org>
 <569FAC90.5030407@oracle.com>
 <alpine.DEB.2.20.1601200954420.23983@east.gentwo.org>
 <20160120212806.GA26965@dhcp22.suse.cz>
 <alpine.DEB.2.20.1601201552590.26496@east.gentwo.org>
 <20160121082402.GA29520@dhcp22.suse.cz>
 <alpine.DEB.2.20.1601210941540.7063@east.gentwo.org>
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List-ID: <linux-mm.kvack.org>
To: Christoph Lameter <cl@linux.com>
Cc: Sasha Levin <sasha.levin@oracle.com>, LKML <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>

On Thu 21-01-16 09:45:12, Christoph Lameter wrote:
> On Thu, 21 Jan 2016, Michal Hocko wrote:
[...]
> > The vmstat update might be still waiting for its timer, idle mode started
> > and kick vmstat_update which might cpumask_test_and_set_cpu. Once the
> > idle terminates and the originally schedule vmstate_update executes it
> > sees the bit set and BUG_ON.
> 
> Ok so we are going into idle mode and the vmstat_update timer is pending.
> Then the timer will not fire since going idle switches preemption off.
> quiet_vmstat will run without the chance of running vmstat_update
> 
> We could be going idle and not have disabled preemption yet. Then
> vmstat_update will run. On return to the idling operation preemption will
> be disabled and quiet_vmstat() will be run.
> 
> I do not see how these two things could race.

It goes like this:
CPU0:						CPU1
vmstat_update
  cpumask_test_and_set_cpu (0->1)
[...]
						vmstat_shepherd
<enter idle>					  cpumask_test_and_clear_cpu(CPU0) (1->0)
quiet_vmstat
  cpumask_test_and_set_cpu (0->1)
  						  queue_delayed_work_on(CPU0)
refresh_cpu_vm_stats()
[...]
vmstat_update
  nothing_to_do
  cpumask_test_and_set_cpu (1->1)
  VM_BUG_ON

Or am I missing something?
-- 
Michal Hocko
SUSE Labs

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