Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

[Peter Zijlstra]

On Wed, Apr 17, 2019 at 01:45:10PM -0400, Waiman Long wrote:
> On 04/17/2019 09:58 AM, Peter Zijlstra wrote:
> > On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
> >> +/*
> >> + * Try to acquire read lock before the reader is put on wait queue.
> >> + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
> >> + * is ongoing.
> >> + */
> >> +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
> >> +{
> >> +	long count = atomic_long_read(&sem->count);
> >> +
> >> +	if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count))
> >> +		return false;
> >> +
> >> +	count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
> >> +	if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) {
> >> +		rwsem_set_reader_owned(sem);
> >> +		lockevent_inc(rwsem_opt_rlock);
> >> +		return true;
> >> +	}
> >> +
> >> +	/* Back out the change */
> >> +	atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
> >> +	return false;
> >> +}
> > Doesn't a cmpxchg 'loop' make more sense here?
> 
> Not really. A cmpxchg loop will have one more correctible failure mode -
> a new reader acquire the lock or a reader owner does an unlock. Failures
> caused by the setting of the handoff bit or writer acquiring the lock
> are the same for both cases. I don't see any advantage in using cmpxchg
> loop.

It depends on how many failures vs successes you have. I was expecting
failure to be the most common case, and then you go from 2 atomics to 1.