[RESEND PATCH v4 0/2] cpuhp: fix some st->target issues

[RESEND PATCH v4 0/2] cpuhp: fix some st->target issues

[Phil Auld]

Fix a couple of cpuhp inconsistencies.

The first prevents target_store() from calling cpu_down() when
target == state which prevents the cpu being incorrectly marked
as dying.  The second just makes the boot cpu have a valid cpuhp
target rather than 0 (CPU_OFFLINE) while being in state
CPU_ONLINE.

A further issue which these two patches don't address is that
the cpuX/online file looks at the device->offline state and can
thus get out of sync with the actual cpuhp state if the cpuhp
target is used to change state.

v3: Added code to make sure st->target == target in the nop case.

v4: Use WARN_ON in the case where state == target but st->target does
not.

Phil Auld (2):
  cpuhp: make target_store() a nop when target == state
  cpuhp: Set cpuhp target for boot cpu

 kernel/cpu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

-- 
2.31.1

[RESEND PATCH v4 1/2] cpuhp: make target_store() a nop when target == state

[Phil Auld]

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>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Fixes: 757c989b9994 ("cpu/hotplug: Make target state writeable")
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/cpu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bbad5e375d3b..979de993f853 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 (WARN_ON(st->target != target))
+		st->target = target;
 out:
 	unlock_device_hotplug();
 	return ret ? ret : count;
-- 
2.31.1

[RESEND PATCH v4 2/2] cpuhp: Set cpuhp target for boot cpu

[Phil Auld]

Since the boot cpu does not go through the hotplug process it ends
up with state == CPUHP_ONLINE but target == CPUHP_OFFLINE.
So set the target to match in boot_cpu_hotplug_init().

Signed-off-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>

---
 kernel/cpu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 979de993f853..3f704a8896b0 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2690,6 +2690,7 @@ void __init boot_cpu_hotplug_init(void)
 	cpumask_set_cpu(smp_processor_id(), &cpus_booted_once_mask);
 #endif
 	this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
+	this_cpu_write(cpuhp_state.target, CPUHP_ONLINE);
 }
 
 /*
-- 
2.31.1

Re: [RESEND PATCH v4 0/2] cpuhp: fix some st->target issues

[Phil Auld]

On Thu, Sep 15, 2022 at 11:37:49AM -0400 Phil Auld wrote:
> Fix a couple of cpuhp inconsistencies.
> 
> The first prevents target_store() from calling cpu_down() when
> target == state which prevents the cpu being incorrectly marked
> as dying.  The second just makes the boot cpu have a valid cpuhp
> target rather than 0 (CPU_OFFLINE) while being in state
> CPU_ONLINE.
> 
> A further issue which these two patches don't address is that
> the cpuX/online file looks at the device->offline state and can
> thus get out of sync with the actual cpuhp state if the cpuhp
> target is used to change state.
> 
> v3: Added code to make sure st->target == target in the nop case.
> 
> v4: Use WARN_ON in the case where state == target but st->target does
> not.
> 
> Phil Auld (2):
>   cpuhp: make target_store() a nop when target == state
>   cpuhp: Set cpuhp target for boot cpu
> 
>  kernel/cpu.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> -- 
> 2.31.1
> 

Pingy McPing-face :)

Peter?  Anyone?   It's really not ideal to have a cpu marked dying when
it isn't actually going down. Please take a look. 

Thanks for your time.

Cheers,
Phil

--