On 4/26/2018 1:39 AM, Peter Zijlstra wrote:
On Wed, Apr 25, 2018 at 02:03:19PM +0530, Gaurav Kohli wrote:
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 5043e74..c5c5184 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data)
}
if (kthread_should_park()) {
+ /*
+ * Serialize against wakeup.
*
* Prior wakeups must complete and later wakeups
* will observe TASK_RUNNING.
*
* This avoids the case where the TASK_RUNNING
* store from ttwu() competes with the
* TASK_PARKED store from kthread_parkme().
*
* If the TASK_PARKED store looses that
* competition, kthread_unpark() will go wobbly.
+ */
+ raw_spin_lock(¤t->pi_lock);
__set_current_state(TASK_RUNNING);
+ raw_spin_unlock(¤t->pi_lock);
preempt_enable();
if (ht->park && td->status == HP_THREAD_ACTIVE) {
BUG_ON(td->cpu != smp_processor_id());
Does that work for you?
We have given patch for testing, usually it takes around 2-3 days for reproduction(we will update for the same).
But looking at this a bit more; don't we have the exact same problem
with the TASK_RUNNING store in the !ht->thread_should_run() case?
Suppose a ttwu() happens concurrently there, it can end up competing
against the TASK_INTERRUPTIBLE store, no?
Of course, that race is not fatal, we'll just end up going around the
loop once again I suppose. Maybe a comment there too?
/*
* A similar race is possible here, but loosing
* the TASK_INTERRUPTIBLE store is harmless and
* will make us go around the loop once more.
*/
Actually instead of race, i am seeing wakeup miss problem which is very rare, if we take case of hotplug thread
Controller Hotplug
Loop start
set_current_state(TASK_INTERRUPTIBLE);
if (kthread_should_park()) { -> fails
Set Should_park
then wake_up
if (!ht->thread_should_run(td->cpu)) {
preempt_enable_no_resched();
schedule(); Again went to schedule(which is very rare to occur,not sure whether it hits)
And of course, I suspect we actually want to use TASK_IDLE, smpboot
threads don't want signals do they? But that probably ought to be a
separate patch.
Yes I agree, we can control race from here as well, Please suggest would below change be any help here:
} else {
__set_current_state(TASK_RUNNING);
preempt_enable();
ht->thread_fn(td->cpu);
+ set_current_state(TASK_INTERRUPTIBLE);
+ schedule();
}
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