Re: [Patch v3] rwsem: fix rwsem_is_locked() bugs

[Amerigo Wang]

David Howells wrote:
> Amerigo Wang <amwang@xxxxxxxxxx> wrote:
>
> >  static inline int rwsem_is_locked(struct rw_semaphore *sem)
> >  {
> > -	return (sem->activity != 0);
> > +	int ret;
> > +
> > +	if (spin_trylock_irq(&sem->wait_lock)) {
> > +		ret = !(list_empty(&sem->wait_list) && sem->activity == 0);
> > +		spin_unlock_irq(&sem->wait_lock);
> > +		return ret;
> > +	}
> > +	return 1;
> >  }
>
> Yep...  This seems a reasonable approach, though I contend that if you're
> holding the spinlock, then sem->wait_list _must_ be empty if sem->activity is
> 0 - so that half of the test is redundant.
>
> sem->activity == 0 and sem->wait_list not being empty is a transitional state
> that can only occur in ups and downgrades whilst they hold the spinlock.


Hmm, yeah...

> > diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
> > index 9df3ca5..234d83f 100644
> > --- a/lib/rwsem-spinlock.c
> > +++ b/lib/rwsem-spinlock.c
> > @@ -78,7 +78,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
> >  
> >  	/* grant an infinite number of read locks to the front of the queue */
> >   dont_wake_writers:
> > -	woken = 0;
> > +	/*
> > +	 * we increase ->activity just to make rwsem_is_locked() happy,
> > +	 * to avoid potential cache line ping-pong, we don't do this
> > +	 * within the following loop.
> > +	 */
> > +	woken = sem->activity++;
> >  	while (waiter->flags & RWSEM_WAITING_FOR_READ) {
> >  		struct list_head *next = waiter->list.next;
> >  
> > @@ -94,7 +99,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
> >  		waiter = list_entry(next, struct rwsem_waiter, list);
> >  	}
> >  
> > -	sem->activity += woken;
> > +	sem->activity = woken;
> >  
> >   out:
> >  	return sem;
>
> This change to __rwsem_do_wake() is all unnecessary - you're defending against
> the test of sem->activity by rwsem_is_locked() - but that now happens with the
> spinlock held.

Ah, yes, I knew this, I kept this just for completeness.
I will remove this part then. :)

THanks!

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