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

[Waiman Long]

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.

Cheers,
Longman