From: Phil Auld <pauld@redhat.com>
To: Valentin Schneider <vschneid@redhat.com>
Cc: linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state
Date: Wed, 1 Jun 2022 11:49:37 -0400 [thread overview]
Message-ID: <YpeKkVVp+/JWuxUi@lorien.usersys.redhat.com> (raw)
In-Reply-To: <xhsmho7zf8auj.mognet@vschneid.remote.csb>
On Mon, May 30, 2022 at 01:27:00PM +0100 Valentin Schneider wrote:
> On 27/05/22 09:22, Phil Auld wrote:
> > On Fri, May 27, 2022 at 10:38:24AM +0100 Valentin Schneider wrote:
> >> On 26/05/22 12:06, Phil Auld wrote:
> >> > writing the current state back in hotplug/target calls cpu_down()
> >> > which will set cpu dying even when it isn't and then nothing will
> >> > ever clear it. A stress test that reads values and writes them back
> >> > for all cpu device files in sysfs will trigger the BUG() in
> >> > select_fallback_rq once all cpus are marked as dying.
> >> >
> >> > kernel/cpu.c::target_store()
> >> > ...
> >> > if (st->state < target)
> >> > ret = cpu_up(dev->id, target);
> >> > else
> >> > ret = cpu_down(dev->id, target);
> >> >
> >> > cpu_down() -> cpu_set_state()
> >> > bool bringup = st->state < target;
> >> > ...
> >> > if (cpu_dying(cpu) != !bringup)
> >> > set_cpu_dying(cpu, !bringup);
> >> >
> >> > Fix this by letting state==target fall through in the target_store()
> >> > conditional.
> >> >
> >>
> >> To go back on my data race paranoia: writes to both cpu$x/online and
> >> cpu$x/hotplug/target are serialized by device_hotplug_lock, and so are the
> >> exported kernel hotplug functions ({add, remove}_cpu()).
> >>
> >> That's not cpu_add_remove_lock as I was looking for, but that's still all
> >> under one lock, so I think we're good. Sorry for that!
> >>
> >
> > Right. This catches it up higher so that we don't get into the code that
> > starts actually changing things. I wonder now in the state == target case
> > if we should make sure st->target == target. With the second patch it's
> > less likely to be needed. Thoughts?
> >
>
> Yeah, you could append a simple:
>
> else
> WARN_ON(st->state != target);
I was thinking more like:
else
if (st->target != target) st->target = target;
Since this is a write to the target field and we are not
doing one of the operations that will set target because
state == target we should make sure target == target. Although
that could have its own issues, I suppose. But as I said
fixing the boot cpu should make it much less likely that
st->target != st->state once we have the hotplug lock.
I don't see how that WARN would ever fire. We're under the lock
and nothing is re-reading the value of st->state anyway. Looks more
like a compiler sanity check :)
Cheers,
Phil
> > Maybe I'll include that if/when I have code to keep cpux/online in sync
> > with st->state and cpu_online_mask.
>
--
next prev parent reply other threads:[~2022-06-01 15:49 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-26 16:06 [PATCH v2 0/2] cpuhp: fix some st->target issues Phil Auld
2022-05-26 16:06 ` [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state Phil Auld
2022-05-27 9:38 ` Valentin Schneider
2022-05-27 13:22 ` Phil Auld
2022-05-30 12:27 ` Valentin Schneider
2022-06-01 15:49 ` Phil Auld [this message]
2022-06-01 16:39 ` Valentin Schneider
2022-05-26 16:06 ` [PATCH v2 2/2] cpuhp: Set cpuhp target for boot cpu Phil Auld
2022-05-27 9:38 ` Valentin Schneider
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