Re: [PATCH RT v2] Fix a lockup in wait_for_completion() and friends

[Peter Zijlstra]

On Thu, May 09, 2019 at 06:19:25PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-05-08 15:57:28 [-0500], miny...@acm.org wrote:

> >  kernel/sched/completion.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
> > index 755a58084978..4f9b4cc0c95a 100644
> > --- a/kernel/sched/completion.c
> > +++ b/kernel/sched/completion.c
> > @@ -70,20 +70,20 @@ do_wait_for_common(struct completion *x,
> >                long (*action)(long), long timeout, int state)
> >  {
> >     if (!x->done) {
> > -           DECLARE_SWAITQUEUE(wait);
> > -
> > -           __prepare_to_swait(&x->wait, &wait);
> 
> you can keep DECLARE_SWAITQUEUE remove just __prepare_to_swait()
> 
> >             do {
> > +                   DECLARE_SWAITQUEUE(wait);
> > +
> >                     if (signal_pending_state(state, current)) {
> >                             timeout = -ERESTARTSYS;
> >                             break;
> >                     }
> > +                   __prepare_to_swait(&x->wait, &wait);
> 
> add this, yes and you are done.
> 
> >                     __set_current_state(state);
> >                     raw_spin_unlock_irq(&x->wait.lock);
> >                     timeout = action(timeout);
> >                     raw_spin_lock_irq(&x->wait.lock);
> > +                   __finish_swait(&x->wait, &wait);
> >             } while (!x->done && timeout);
> > -           __finish_swait(&x->wait, &wait);
> >             if (!x->done)
> >                     return timeout;
> >     }

Now.. that will fix it, but I think it is also wrong.

The problem being that it violates FIFO, something that might be more
important on -RT than elsewhere.

The regular wait API seems confused/inconsistent when it uses
autoremove_wake_function and default_wake_function, which doesn't help,
but we can easily support this with swait -- the problematic thing is
the custom wake functions, we musn't do that.

(also, mingo went and renamed a whole bunch of wait_* crap and didn't do
the same to swait_ so now its named all different :/)

Something like the below perhaps.

---
diff --git a/include/linux/swait.h b/include/linux/swait.h
index 73e06e9986d4..f194437ae7d2 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -61,11 +61,13 @@ struct swait_queue_head {
 struct swait_queue {
        struct task_struct      *task;
        struct list_head        task_list;
+       unsigned int            remove;
 };
 
 #define __SWAITQUEUE_INITIALIZER(name) {                               \
        .task           = current,                                      \
        .task_list      = LIST_HEAD_INIT((name).task_list),             \
+       .remove         = 1,                                            \
 }
 
 #define DECLARE_SWAITQUEUE(name)                                       \
diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
index e83a3f8449f6..86974ecbabfc 100644
--- a/kernel/sched/swait.c
+++ b/kernel/sched/swait.c
@@ -28,7 +28,8 @@ void swake_up_locked(struct swait_queue_head *q)
 
        curr = list_first_entry(&q->task_list, typeof(*curr), task_list);
        wake_up_process(curr->task);
-       list_del_init(&curr->task_list);
+       if (curr->remove)
+               list_del_init(&curr->task_list);
 }
 EXPORT_SYMBOL(swake_up_locked);
 
@@ -57,7 +58,8 @@ void swake_up_all(struct swait_queue_head *q)
                curr = list_first_entry(&tmp, typeof(*curr), task_list);
 
                wake_up_state(curr->task, TASK_NORMAL);
-               list_del_init(&curr->task_list);
+               if (curr->remove)
+                       list_del_init(&curr->task_list);
 
                if (list_empty(&tmp))
                        break;