Re: [PATCH 1/2] scalable rw_mutex

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sat, 2007-05-12 at 20:04 +0400, Oleg Nesterov wrote:
> On 05/12, Peter Zijlstra wrote:
> >
> > +static inline void rw_mutex_readers_dec(struct rw_mutex *rw_mutex)
> > +{
> > +	percpu_counter_dec(&rw_mutex->readers);
> > +	smp_wmb();
> > +}
> >
> > +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
> > +{
> > +	rw_mutex_readers_dec(rw_mutex);
> > +	/*
> > +	 * on the slow path;
> > +	 * nudge the writer waiting for the last reader to go away
> > +	 */
> > +	if (unlikely(rw_mutex_reader_slow(rw_mutex)))
> > +		rw_mutex_writer_wake(rw_mutex);
> > +}
> >
> > +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> > +{
> > +	mutex_lock_nested(&rw_mutex->write_mutex, subclass);
> > +
> > +	/*
> > +	 * block new readers
> > +	 */
> > +	mutex_lock_nested(&rw_mutex->read_mutex, subclass);
> > +	rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> > +	/*
> > +	 * and wait for all current readers to go away
> > +	 */
> > +	rw_mutex_writer_wait(rw_mutex, (rw_mutex_readers(rw_mutex) == 0));
> > +}
> 
> I think this is still not right, but when it comes to barriers we
> need a true expert (Paul cc-ed).
> 
> this code roughly does (the only reader does unlock)
> 
> 	READER			WRITER
> 
> 	readers = 0;		state = 1;
> 	wmb();			wmb();
> 	CHECK(state != 0)	CHECK(readers == 0)
> 
> We need to ensure that we can't miss both CHECKs. Either reader
> should see RW_MUTEX_READER_SLOW, o writer sees "readers == 0"
> and does not sleep.
> 
> In that case both barriers should be converted to smp_mb(). There
> was a _long_ discussion about STORE-MB-LOAD behaviour, and experts
> seem to believe everething is ok.

Ah, but note that both those CHECK()s have a rmb(), so that ends up
being:

	READER				WRITER

	readers = 0;			state = 1;
	wmb();				wmb();

	rmb();				rmb();		
	if (state != 0)			if (readers == 0)

and a wmb+rmb is a full mb, right?

> Another question. Isn't it possible to kill rw_mutex->status ?
> 
> I have a vague feeling you can change the code so that
> 
> 	rw_mutex_reader_slow() <=> "->waiter != NULL"
> 
> , but I am not sure.

If not now, it might be possible to make it so. Thanks for the
suggestion.

Peter