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SubjectRe: [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG
On 2018-06-27 08:36:57 [-0400], Steven Rostedt wrote:
> On Wed, Apr 11, 2018 at 09:07:30PM +0200, Sebastian Andrzej Siewior wrote:
> >
> > This already happens:
> > - vmstat_shepherd() does get_online_cpus() and within this block it does
> > queue_delayed_work_on(). So this has to wait until cpuhotplug
> > completed before it can schedule something and then it won't schedule
> > anything on the "off" CPU.
>
> But can't we have something like this happen: ?
>
> CPU0 CPU1 CPU2
> ---- ---- ----
> get_online_cpus()
> queue_work(vmstat_update, cpu1)
> wakeup(kworker/1)
> High prio task running
> put_online_cpus()
> Shutdown CPU 1
> migrate kworker/1
> schedule kworker/1
> (smp_processor_id() != 1)

I don't get CPU1 doing in here. kworker/1 should not be migrated to
begin with. The work item should be done before CPU2 shutdowns CPU1 (or
0) because vmstat_cpu_down_prep() cancels the work while the CPUs goes
down so it should not be migrated. Also, the work is enqueued while
holding the CPU HP lock.

Sebastian

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