Re: [RFC] locking: rwbase: Take care of ordering guarantee for fastpath reader

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Sep 01, 2021 at 01:22:42PM -0700, Davidlohr Bueso wrote:
> On Wed, 01 Sep 2021, Boqun Feng wrote:
> > diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
> > index 4ba15088e640..a1886fd8bde6 100644
> > --- a/kernel/locking/rwbase_rt.c
> > +++ b/kernel/locking/rwbase_rt.c
> > @@ -41,6 +41,12 @@
> >  * The risk of writer starvation is there, but the pathological use cases
> >  * which trigger it are not necessarily the typical RT workloads.
> >  *
> > + * Fast-path orderings:
> > + * The lock/unlock of readers can run in fast paths: lock and unlock are only
> > + * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
> > + * semantics of rwbase_rt. Atomic ops then should be stronger than _acquire()
> > + * and _release() to provide necessary ordering guarantee.
> 
> Perhaps the following instead?
> 

Thanks.

> + * Ordering guarantees: As with any locking primitive, (load)-ACQUIRE and
> + * (store)-RELEASE semantics are guaranteed for lock and unlock operations,
> + * respectively; such that nothing leaks out of the critical region. When
> + * writers are involved this is provided through the rtmutex. However, for
> + * reader fast-paths, the atomics provide at least such guarantees.
> 

However, this is a bit inaccurate, yes, writers always acquire the lock
(->readers) in the critical sections of ->wait_lock. But if readers run
the fast-paths, the atomics of the writers have to provide the ordering,
because we can rely on rtmutex orderings only if both sides run in
slow-paths.

> Also, I think you could remove most of the comments wrt _acquire or _release
> in the fastpath for each ->readers atomic op, unless it isn't obvious.
> 
> > + *
> >  * Common code shared between RT rw_semaphore and rwlock
> >  */
> > 
> > @@ -53,6 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
> > 	 * set.
> > 	 */
> > 	for (r = atomic_read(&rwb->readers); r < 0;) {
> 
> Unrelated, but we probably wanna get rid of these abusing for-loops throughout.
> 

Agreed, let me see what I can do.

> > +		/* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */
> > 		if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
> 
> As Waiman suggested, this can be _acquire() - albeit we're only missing
> an L->L for acquire semantics upon returning, per the control dependency
> already guaranteeing L->S. That way we would loop with _relaxed().
> 

_acquire() is fine, I think. But probably a separate patch. We should be
careful when relaxing the ordering, and perhaps, with some performance
numbers showing the benefits.

> > 			return 1;
> > 	}
> > @@ -162,6 +169,8 @@ static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
> > 	/*
> > 	 * rwb->readers can only hit 0 when a writer is waiting for the
> > 	 * active readers to leave the critical section.
> > +	 *
> > +	 * dec_and_test() is fully ordered, provides RELEASE.
> > 	 */
> > 	if (unlikely(atomic_dec_and_test(&rwb->readers)))
> > 		__rwbase_read_unlock(rwb, state);
> > @@ -172,7 +181,11 @@ static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
> > {
> > 	struct rt_mutex_base *rtm = &rwb->rtmutex;
> > 
> > -	atomic_add(READER_BIAS - bias, &rwb->readers);
> > +	/*
> > +	 * _release() is needed in case that reader is in fast path, pairing
> > +	 * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE
> > +	 */
> > +	(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
> 
> Hmmm while defined, there are no users atomic_add_return_release (yet?). I think

There is a usage of atomic_sub_return_release() in queued_spin_unlock()
;-)

> this is because the following is preferred when the return value is not really
> wanted, but only the Rmw ordering it provides:
> 
> +       smp_mb__before_atomic(); /* provide RELEASE semantics */
> 	atomic_add(READER_BIAS - bias, &rwb->readers);
> 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> 	rwbase_rtmutex_unlock(rtm);
> 

smp_mb__before_atomic() + atomic will be a smp_mb() + atomic on weakly
ordered archs (e.g. ARM64 and PowerPC), while atomic_*_return_release()
will be a release atomic operation (e.g. ldxr/stxlr on ARM64), the
latter is considered more cheap.

Regards,
Boqun

> > 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
> > 	rwbase_rtmutex_unlock(rtm);
> > }
> 
> Thanks,
> Davidlohr