Re: [PATCH v11 1/4] qrwlock: A queue read/write lock implementation

[Waiman Long <waiman.long@hp.com>]

On 01/24/2014 03:25 AM, Peter Zijlstra wrote:
> On Thu, Jan 23, 2014 at 11:28:48PM -0500, Waiman Long wrote:
>> +/**
>> + * queue_read_trylock - try to acquire read lock of a queue rwlock
>> + * @lock : Pointer to queue rwlock structure
>> + * Return: 1 if lock acquired, 0 if failed
>> + */
>> +static inline int queue_read_trylock(struct qrwlock *lock)
>> +{
>> +	union qrwcnts cnts;
>> +
>> +	cnts.rwc = ACCESS_ONCE(lock->cnts.rwc);
>> +	if (likely(!cnts.writer)) {
>> +		cnts.rwc = (u32)atomic_add_return(_QR_BIAS,&lock->cnts.rwa);
>> +		if (likely(!cnts.writer)) {
>> +			smp_mb__after_atomic_inc();
> That's superfluous, as atomic_add_return() is documented as being a full
> barrier.

Yes, you are right. I have reviewed the memory_barrier.txt again and 
atomic_add_return() is supposed to act as a memory barrier. So no extra 
barrier. I will correct that in the next version.

>> +			return 1;
>> +		}
>> +		atomic_sub(_QR_BIAS,&lock->cnts.rwa);
>> +	}
>> +	return 0;
>> +}
>> +
>> +/**
>> + * queue_write_trylock - try to acquire write lock of a queue rwlock
>> + * @lock : Pointer to queue rwlock structure
>> + * Return: 1 if lock acquired, 0 if failed
>> + */
>> +static inline int queue_write_trylock(struct qrwlock *lock)
>> +{
>> +	union qrwcnts old, new;
>> +
>> +	old.rwc = ACCESS_ONCE(lock->cnts.rwc);
>> +	if (likely(!old.rwc)) {
>> +		new.rwc = old.rwc;
>> +		new.writer = _QW_LOCKED;
>> +		if (likely(cmpxchg(&lock->cnts.rwc, old.rwc, new.rwc)
>> +				== old.rwc))
> One could actually use atomic_cmpxchg() and avoid one (ab)use of that
> union :-)

I think either one is fine. I would like to keep the original code if it 
is not really a problem.

>> +			return 1;
>> +	}
>> +	return 0;
>> +}
>> +/**
>> + * queue_read_lock - acquire read lock of a queue rwlock
>> + * @lock: Pointer to queue rwlock structure
>> + */
>> +static inline void queue_read_lock(struct qrwlock *lock)
>> +{
>> +	union qrwcnts cnts;
>> +
>> +	cnts.rwc = atomic_add_return(_QR_BIAS,&lock->cnts.rwa);
>> +	if (likely(!cnts.writer)) {
>> +		smp_mb__after_atomic_inc();
> Superfluous again.

Will remove that.

>> +		return;
>> +	queue_write_lock_slowpath(lock);
>> +}
>> +
>> +/**
>> + * queue_read_unlock - release read lock of a queue rwlock
>> + * @lock : Pointer to queue rwlock structure
>> + */
>> +static inline void queue_read_unlock(struct qrwlock *lock)
>> +{
>> +	/*
>> +	 * Atomically decrement the reader count
>> +	 */
>> +	smp_mb__before_atomic_dec();
>> +	atomic_sub(_QR_BIAS,&lock->cnts.rwa);
>> +}
>> +
>> +/**
>> + * queue_write_unlock - release write lock of a queue rwlock
>> + * @lock : Pointer to queue rwlock structure
>> + */
>> +static inline void queue_write_unlock(struct qrwlock *lock)
>> +{
>> +	/*
>> +	 * If the writer field is atomic, it can be cleared directly.
>> +	 * Otherwise, an atomic subtraction will be used to clear it.
>> +	 */
>> +	if (__native_word(lock->cnts.writer))
>> +		smp_store_release(&lock->cnts.writer, 0);
>> +	else {
>> +		smp_mb__before_atomic_dec();
>> +		atomic_sub(_QW_LOCKED,&lock->cnts.rwa);
>> +	}
> Missing {}, Documentation/CodingStyle Chapter 3 near the very end.

Thank for spotting that. Will fix it in the next version.

-Longman