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

[Peter Zijlstra]

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);