Re: [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG
From: Sebastian Andrzej Siewior
Date: Wed Apr 18 2018 - 11:45:06 EST
any reason not to accept the revert?
On 2018-04-11 21:07:29 [+0200], To Tejun Heo wrote:
> On 2018-04-11 16:42:21 [+0200], To Tejun Heo wrote:
> > > > So is this perhaps related to the cpu hotplug that  mentions? e.g. is
> > > > the cpu being hotplugged cpu 1, the worker started too early before
> > > > stuff can be scheduled on the CPU, so it has to run on different than
> > > > designated CPU?
> > > >
> > > >  https://marc.info/?l=linux-mm&m=152088260625433&w=2
> > >
> > > The report says that it happens when hotplug is attempted. Per-cpu
> > > doesn't pin the cpu alive, so if the cpu goes down while a work item
> > > is in flight or a work item is queued while a cpu is offline it'll end
> > > up executing on some other cpu. So, if a piece of code doesn't want
> > > that happening, it gotta interlock itself - ie. start queueing when
> > > the cpu comes online and flush and prevent further queueing when its
> > > cpu goes down.
> > I missed that cpuhotplug part while reading it. So in that case, let me
> > add a CPU-hotplug notifier which cancels that work. After all it is not
> > need once the CPU is gone.
> 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.
> - The work item itself (vmstat_update()) schedules itself
> (conditionally) again.
> - vmstat_cpu_down_prep() is the down event and does
> cancel_delayed_work_sync(). So it waits for the work-item to complete
> and cancels it.
> This looks all good to me.
> > > Thanks.