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

[Peter Zijlstra]

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.

> +			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 :-)

> +			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.

> +		return;
> +	}
> +	/*
> +	 * Slowpath will decrement the reader count, if necessary
> +	 */
> +	queue_read_lock_slowpath(lock);
> +}
> +
> +/**
> + * queue_write_lock - acquire write lock of a queue rwlock
> + * @lock : Pointer to queue rwlock structure
> + */
> +static inline void queue_write_lock(struct qrwlock *lock)
> +{
> +	/*
> +	 * Optimize for the unfair lock case where the fair flag is 0.
> +	 */
> +	if (cmpxchg(&lock->cnts.rwc, 0, _QW_LOCKED) == 0)

Could equally be atomic_cmpxchg() again.

> +		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.

> +}
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