Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 18, 2019 at 12:45:47PM +0100, Will Deacon wrote:
> On Thu, Jul 18, 2019 at 12:58:12PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 18, 2019 at 10:26:41AM +0100, Will Deacon wrote:
> > 
> > > /*
> > >  * We need to ensure ACQUIRE semantics when reading sem->count so that
> > >  * we pair with the RELEASE store performed by an unlocking/downgrading
> > >  * writer.
> > >  *
> > >  * P0 (writer)			P1 (reader)
> > >  *
> > >  * down_write(sem);
> > >  * <write shared data>
> > >  * downgrade_write(sem);
> > >  * -> fetch_add_release(&sem->count)
> > >  *
> > >  *				down_read_slowpath(sem);
> > >  *				-> atomic_read(&sem->count)
> > >  *				   <ctrl dep>
> > >  *				   smp_acquire__after_ctrl_dep()
> > >  *				<read shared data>
> > >  */
> > 
> > So I'm thinking all this is excessive; the simple rule is: lock acquire
> > should imply ACQUIRE, we all know why.
> 
> Fair enough, I just thought this was worth highlighting because you can't
> reply on the wait_lock to give you ACQUIRE ordering.

Right, not in this case, because sem->count is not fully serialized by
it, whereas below the wait-queue is.

> > ---
> > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> > index 37524a47f002..9eb630904a17 100644
> > --- a/kernel/locking/rwsem.c
> > +++ b/kernel/locking/rwsem.c
> > @@ -1000,6 +1000,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
> >  	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> >  	adjustment = 0;
> >  	if (rwsem_optimistic_spin(sem, false)) {
> > +		/* rwsem_optimistic_spin() implies ACQUIRE through rwsem_*trylock() */
> 
> I couldn't figure out if this was dependent on the return value or not,

I went with the fact that the only way to return true is if taken
becomes true; and that only happens through
rwsem_try_{read,write}_lock_unqueued(), and both imply ACQUIRE on
success.

> and looking at osq_lock() I also couldn't see the ACQUIRE barrier when we're
> spinning on node->locked. Hmm.

Yes, osq is not a full lock and does not imply these barriers. This came
up somewhere, did we forget to write a comment on that? Lemme go look.

> >  		/*
> >  		 * Wake up other readers in the wait list if the front
> >  		 * waiter is a reader.
> > @@ -1014,6 +1015,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
> >  		}
> >  		return sem;
> >  	} else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
> > +		/* rwsem_reader_phase_trylock() implies ACQUIRE */
> 
> Can we add "on success" to the end of this, please?

Good point.

> >  		return sem;
> >  	}
> >  
> > @@ -1032,6 +1034,8 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
> >  		 */
> >  		if (adjustment && !(atomic_long_read(&sem->count) &
> >  		     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> > +			/* Provide lock ACQUIRE */
> > +			smp_acquire__after_ctrl_dep();
> >  			raw_spin_unlock_irq(&sem->wait_lock);
> >  			rwsem_set_reader_owned(sem);
> >  			lockevent_inc(rwsem_rlock_fast);
> > @@ -1065,15 +1069,25 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
> >  	wake_up_q(&wake_q);
> >  
> >  	/* wait to be given the lock */
> > -	while (true) {
> > +	for (;;) {
> >  		set_current_state(state);
> > -		if (!waiter.task)
> > +		if (!smp_load_acquire(&waiter.task)) {
> > +			/*
> > +			 * Matches rwsem_mark_wake()'s smp_store_release() and ensures
> > +			 * we're ordered against its sem->count operations.
> > +			 */
> >  			break;
> > +		}
> 
> Ack. Also, grepping for 'waiter.task' reveals a similar usage in
> drivers/tty/tty_ldsem.c if you're feeling brave enough.

*sigh* of course, for every bug there needs to be a second copy
somewhere.

I'll go look there too. Thanks!