Messages in this thread | | | Date | Fri, 24 Jan 2014 23:39:10 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v11 1/4] qrwlock: A queue read/write lock implementation |
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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
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