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From:   Phil Auld <pauld@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Valentin Schneider <vschneid@redhat.com>
Subject: [PATCH v3 1/2] cpuhp: make target_store() a nop when target == state
Date:   Mon, 11 Jul 2022 17:16:18 -0400
Message-Id: <20220711211619.112854-2-pauld@redhat.com>
In-Reply-To: <20220711211619.112854-1-pauld@redhat.com>
References: <20220711211619.112854-1-pauld@redhat.com>
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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. Also make sure st->target == target in that case.

Signed-off-by: Phil Auld <pauld@redhat.com>
---
 kernel/cpu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bbad5e375d3b..305694a2ca26 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2326,8 +2326,10 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
 
 	if (st->state < target)
 		ret = cpu_up(dev->id, target);
-	else
+	else if (st->state > target)
 		ret = cpu_down(dev->id, target);
+	else if (st->target != target)
+		st->target = target;
 out:
 	unlock_device_hotplug();
 	return ret ? ret : count;
-- 
2.31.1