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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