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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 21 Jan 2016 09:45:12 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Michal Hocko <mhocko@...nel.org>
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 Thu, 21 Jan 2016, Michal Hocko wrote:

> > > 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.
> >
> >
> > Hmmm... the vmstat_update can be interrupted while running and the cpu put
> > into idle mode? If vmstat_update is running then the cpu is not idle but
> > running code. If this is really going on then there is other stuff wrong
> > with the idling logic.
>
> 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.