On Tue, May 01, 2018 at 12:18:45PM +0200, Peter Zijlstra wrote:
> Aaaah... I think I've spotted a problem there. We clear SHOULD_PARK
> before we rebind, so if the thread lost the first PARKED store,
> does the completion, gets migrated, cycles through the loop and now
> observes !SHOULD_PARK and bails the wait-loop, then __kthread_bind()
> will forever wait.
Something like so perhaps...
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -451,6 +451,21 @@ void kthread_unpark(struct task_struct *
{
struct kthread *kthread = to_kthread(k);
+ if (test_bit(KTHREAD_IS_PARKED)) {
+ /*
+ * Newly created kthread was parked when the CPU was offline.
+ * The binding was lost and we need to set it again.
+ */
+ if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
+ __kthread_bind(k, kthread->cpu, TASK_PARKED);
+ }
+
+ /*
+ * Ensures the IS_PARKED load precedes the !SHOULD_PARK store.
+ * matched by the smp_mb() from test_and_set_bit() in __kthread_parkme().
+ */
+ smp_mb__before_atomic();
+
clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
/*
* We clear the IS_PARKED bit here as we don't wait
@@ -458,15 +473,8 @@ void kthread_unpark(struct task_struct *
* park before that happens we'd see the IS_PARKED bit
* which might be about to be cleared.
*/
- if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
- /*
- * Newly created kthread was parked when the CPU was offline.
- * The binding was lost and we need to set it again.
- */
- if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
- __kthread_bind(k, kthread->cpu, TASK_PARKED);
+ if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags))
wake_up_state(k, TASK_PARKED);
- }
}
EXPORT_SYMBOL_GPL(kthread_unpark);
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