Re: [PATCH] kernel/locking/semaphore: use wake_q in up()

[Daniel Vetter]

On Fri, May 10, 2019 at 11:28 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Thu 2019-05-09 22:06:33, Daniel Vetter wrote:
> > console_trylock, called from within printk, can be called from pretty
> > much anywhere. Including try_to_wake_up. Note that this isn't common,
> > usually the box is in pretty bad shape at that point already. But it
> > really doesn't help when then lockdep jumps in and spams the logs,
> > potentially obscuring the real backtrace we're really interested in.
> > One case I've seen (slightly simplified backtrace):
> >
> > Fix this specific locking recursion by moving the wake_up_process out
> > from under the semaphore.lock spinlock, using wake_q as recommended by
> > Peter Zijlstra.
>
> It might make sense to mention also the optimization effect mentioned
> by Peter.
>
> > diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> > index 561acdd39960..7a6f33715688 100644
> > --- a/kernel/locking/semaphore.c
> > +++ b/kernel/locking/semaphore.c
> > @@ -169,6 +169,14 @@ int down_timeout(struct semaphore *sem, long timeout)
> >  }
> >  EXPORT_SYMBOL(down_timeout);
> >
> > +/* Functions for the contended case */
> > +
> > +struct semaphore_waiter {
> > +     struct list_head list;
> > +     struct task_struct *task;
> > +     bool up;
> > +};
> > +
> >  /**
> >   * up - release the semaphore
> >   * @sem: the semaphore to release
> > @@ -179,24 +187,25 @@ EXPORT_SYMBOL(down_timeout);
> >  void up(struct semaphore *sem)
> >  {
> >       unsigned long flags;
> > +     struct semaphore_waiter *waiter;
> > +     DEFINE_WAKE_Q(wake_q);
>
> We need to call wake_q_init(&wake_q) to make sure that
> it is empty.

DEFINE_WAKE_Q does that already, and if it didn't, I'd wonder how I
managed to boot with this patch. console_lock is usally terribly
contented because thanks to fbcon we must do a full display modeset
while holding it, which takes forever. As long as anyone printks
meanwhile (guaranteed while loading drivers really) you have
contention.
-Daniel


> Best Regards,
> Petr
>
> >       raw_spin_lock_irqsave(&sem->lock, flags);
> > -     if (likely(list_empty(&sem->wait_list)))
> > +     if (likely(list_empty(&sem->wait_list))) {
> >               sem->count++;
> > -     else
> > -             __up(sem);
> > +     } else {
> > +             waiter =  list_first_entry(&sem->wait_list,
> > +                                        struct semaphore_waiter, list);
> > +             list_del(&waiter->list);
> > +             waiter->up = true;
> > +             wake_q_add(&wake_q, waiter->task);
> > +     }
> >       raw_spin_unlock_irqrestore(&sem->lock, flags);
> > +
> > +     wake_up_q(&wake_q);
> >  }
> >  EXPORT_SYMBOL(up);
> >
> > -/* Functions for the contended case */
> > -
> > -struct semaphore_waiter {
> > -     struct list_head list;
> > -     struct task_struct *task;
> > -     bool up;
> > -};
> > -
> >  /*
> >   * Because this function is inlined, the 'state' parameter will be
> >   * constant, and thus optimised away by the compiler.  Likewise the



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch