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Date:   Tue, 1 May 2018 15:19:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Kohli, Gaurav" <gkohli@codeaurora.org>
Cc:     tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org,
        bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org,
        Neeraj Upadhyay <neeraju@codeaurora.org>,
        Will Deacon <will.deacon@arm.com>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup
Message-ID: <20180501131904.GG12217@hirez.programming.kicks-ass.net>
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> On 5/1/2018 5:01 PM, Peter Zijlstra wrote:

> > Let me ponder what the best solution is, it's a bit of a mess.

So there's:

 - TASK_PARKED, which we can make a special state; this solves the
   problem because then wait_task_inactive() is guaranteed to see
   TASK_PARKED and DTRT.

 - complete(&kthread->parked), which we can do inside schedule(); this
   solves the problem because then kthread_park() will not return early
   and the task really is blocked.

and I'm fairly sure I thought of a 3rd way to cure things, but now that
I'm writing things down I cannot seem to remember :/ -- could be we muck
with wait_task_inactive().

In any case, I hate all of them, but I think the completion one is the
least horrible because it gives the strongest guarantees and cleans up
most. But promoting TASK_PARKED to special certainly is the earier
patch.

The below boots, but that's about all I did with it. Opinions?

---

--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -62,6 +62,7 @@ void *kthread_probe_data(struct task_str
 int kthread_park(struct task_struct *k);
 void kthread_unpark(struct task_struct *k);
 void kthread_parkme(void);
+void kthread_park_complete(struct task_struct *k);
 
 int kthreadd(void *unused);
 extern struct task_struct *kthreadd_task;
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -55,7 +55,6 @@ enum KTHREAD_BITS {
 	KTHREAD_IS_PER_CPU = 0,
 	KTHREAD_SHOULD_STOP,
 	KTHREAD_SHOULD_PARK,
-	KTHREAD_IS_PARKED,
 };
 
 static inline void set_kthread_struct(void *kthread)
@@ -177,14 +176,12 @@ void *kthread_probe_data(struct task_str
 
 static void __kthread_parkme(struct kthread *self)
 {
-	__set_current_state(TASK_PARKED);
-	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
-		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
-			complete(&self->parked);
+	for (;;) {
+		set_current_state(TASK_PARKED);
+		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
+			break;
 		schedule();
-		__set_current_state(TASK_PARKED);
 	}
-	clear_bit(KTHREAD_IS_PARKED, &self->flags);
 	__set_current_state(TASK_RUNNING);
 }
 
@@ -194,6 +191,11 @@ void kthread_parkme(void)
 }
 EXPORT_SYMBOL_GPL(kthread_parkme);
 
+void kthread_park_complete(struct task_struct *k)
+{
+	complete(&to_kthread(k)->parked);
+}
+
 static int kthread(void *_create)
 {
 	/* Copy data: it's on kthread's stack */
@@ -450,22 +452,15 @@ void kthread_unpark(struct task_struct *
 {
 	struct kthread *kthread = to_kthread(k);
 
-	clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
 	/*
-	 * We clear the IS_PARKED bit here as we don't wait
-	 * until the task has left the park code. So if we'd
-	 * park before that happens we'd see the IS_PARKED bit
-	 * which might be about to be cleared.
+	 * Newly created kthread was parked when the CPU was offline.
+	 * The binding was lost and we need to set it again.
 	 */
-	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);
-		wake_up_state(k, TASK_PARKED);
-	}
+	if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
+		__kthread_bind(k, kthread->cpu, TASK_PARKED);
+
+	clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+	wake_up_state(k, TASK_PARKED);
 }
 EXPORT_SYMBOL_GPL(kthread_unpark);
 
@@ -488,12 +483,13 @@ int kthread_park(struct task_struct *k)
 	if (WARN_ON(k->flags & PF_EXITING))
 		return -ENOSYS;
 
-	if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
-		set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
-		if (k != current) {
-			wake_up_process(k);
-			wait_for_completion(&kthread->parked);
-		}
+	if (WARN_ON_ONCE(test_bit(KTHREAD_SHOULD_PARK, &kthread->flags)))
+		return -EBUSY;
+
+	set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+	if (k != current) {
+		wake_up_process(k);
+		wait_for_completion(&kthread->parked);
 	}
 
 	return 0;
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7,6 +7,8 @@
  */
 #include "sched.h"
 
+#include <linux/kthread.h>
+
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
 
@@ -2718,20 +2720,28 @@ static struct rq *finish_task_switch(str
 		membarrier_mm_sync_core_before_usermode(mm);
 		mmdrop(mm);
 	}
-	if (unlikely(prev_state == TASK_DEAD)) {
-		if (prev->sched_class->task_dead)
-			prev->sched_class->task_dead(prev);
+	if (unlikely(prev_state & (TASK_DEAD|TASK_PARKED))) {
+		switch (prev_state) {
+		case TASK_DEAD:
+			if (prev->sched_class->task_dead)
+				prev->sched_class->task_dead(prev);
+
+			/*
+			 * Remove function-return probe instances associated with this
+			 * task and put them back on the free list.
+			 */
+			kprobe_flush_task(prev);
 
-		/*
-		 * Remove function-return probe instances associated with this
-		 * task and put them back on the free list.
-		 */
-		kprobe_flush_task(prev);
+			/* Task is done with its stack. */
+			put_task_stack(prev);
 
-		/* Task is done with its stack. */
-		put_task_stack(prev);
+			put_task_struct(prev);
+			break;
 
-		put_task_struct(prev);
+		case TASK_PARKED:
+			kthread_park_complete(prev);
+			break;
+		}
 	}
 
 	tick_nohz_task_switch();