Re: [patch V2 9/9] tasklets: Prevent kill/unlock_wait deadlock on RT

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 04, 2020 at 06:02:00PM +0100, Thomas Gleixner wrote:
> @@ -825,7 +848,20 @@ void tasklet_kill(struct tasklet_struct
>  
>  	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
>  		do {
> -			yield();
>  		} while (test_bit(TASKLET_STATE_SCHED, &t->state));
>  	}
>  	tasklet_unlock_wait(t);


Egads... should we not start by doing something like this?


---
diff --git a/kernel/softirq.c b/kernel/softirq.c
index d5bfd5e661fc..95ff5b7f1833 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -529,6 +529,16 @@ void __tasklet_hi_schedule(struct tasklet_struct *t)
 }
 EXPORT_SYMBOL(__tasklet_hi_schedule);
 
+static inline bool tasklet_clear_sched(struct tasklet_struct *t)
+{
+	if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state)) {
+		wake_up_var(&t->state);
+		return true;
+	}
+
+	return false;
+}
+
 static void tasklet_action_common(struct softirq_action *a,
 				  struct tasklet_head *tl_head,
 				  unsigned int softirq_nr)
@@ -548,8 +558,7 @@ static void tasklet_action_common(struct softirq_action *a,
 
 		if (tasklet_trylock(t)) {
 			if (!atomic_read(&t->count)) {
-				if (!test_and_clear_bit(TASKLET_STATE_SCHED,
-							&t->state))
+				if (!tasklet_clear_sched(t))
 					BUG();
 				if (t->use_callback)
 					t->callback(t);
@@ -609,13 +618,11 @@ void tasklet_kill(struct tasklet_struct *t)
 	if (in_interrupt())
 		pr_notice("Attempt to kill tasklet from interrupt\n");
 
-	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
-		do {
-			yield();
-		} while (test_bit(TASKLET_STATE_SCHED, &t->state));
-	}
+	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state))
+		wait_var_event(&t->state, !test_bit(TASKLET_STATE_SCHED, &t->state));
+
 	tasklet_unlock_wait(t);
-	clear_bit(TASKLET_STATE_SCHED, &t->state);
+	tasklet_clear_sched(t);
 }
 EXPORT_SYMBOL(tasklet_kill);