From: Valentin Schneider <vschneid@redhat.com>
To: Phil Auld <pauld@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: Mon, 30 May 2022 13:27:00 +0100 [thread overview]
Message-ID: <xhsmho7zf8auj.mognet@vschneid.remote.csb> (raw)
In-Reply-To: <20220527132156.GB26124@pauld.bos.csb>
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);
> 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:[~2022-05-30 12:27 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 [this message]
2022-06-01 15:49 ` Phil Auld
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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