Messages in this thread | | | Date | Wed, 23 Oct 2013 12:58:10 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v4 1/3] qrwlock: A queue read/write lock implementation |
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On 10/23/2013 08:00 AM, walken@google.com wrote: > On Wed, Oct 02, 2013 at 10:09:04AM -0400, Waiman Long wrote: >> >> Tim Chen also tested the qrwlock with Ingo's patch on a 4-socket >> machine. It was found the performance improvement of 11% was the >> same with regular rwlock or queue rwlock. >> >> Signed-off-by: Waiman Long<Waiman.Long@hp.com> > I haven't followed all the locking threads lately; did this get into any > tree yet and is it still being considered ?
I think it is still being considered. I am hoping that it can get into 3.13, if possible.
>> + * Writer state values& mask >> + */ >> +#define QW_WAITING 1 /* A writer is waiting */ >> +#define QW_LOCKED 0xff /* A writer holds the lock */ >> +#define QW_MASK_FAIR ((u8)~QW_WAITING) /* Mask for fair reader */ >> +#define QW_MASK_UNFAIR ((u8)~0) /* Mask for unfair reader */ > I'm confused - I expect fair readers want to queue behind a waiting writer, > so shouldn't this be QW_MASK_FAIR=~0 and QW_MASK_UNFAIR=~QW_WAITING ?
Yes, you are right. I think I had mixed up the values in a revision to the patch. I will send out an updated patch with the right values.
>> +/** >> + * wait_in_queue - Add to queue and wait until it is at the head >> + * @lock: Pointer to queue rwlock structure >> + * @node: Node pointer to be added to the queue >> + * >> + * The use of smp_wmb() is to make sure that the other CPUs see the change >> + * ASAP. >> + */ >> +static __always_inline void >> +wait_in_queue(struct qrwlock *lock, struct qrwnode *node) >> +{ >> + struct qrwnode *prev; >> + >> + node->next = NULL; >> + node->wait = true; >> + prev = xchg(&lock->waitq, node); >> + if (prev) { >> + prev->next = node; >> + smp_wmb(); >> + /* >> + * Wait until the waiting flag is off >> + */ >> + while (ACCESS_ONCE(node->wait)) >> + cpu_relax(); >> + } >> +} >> + >> +/** >> + * signal_next - Signal the next one in queue to be at the head >> + * @lock: Pointer to queue rwlock structure >> + * @node: Node pointer to the current head of queue >> + */ >> +static __always_inline void >> +signal_next(struct qrwlock *lock, struct qrwnode *node) >> +{ >> + struct qrwnode *next; >> + >> + /* >> + * Try to notify the next node first without disturbing the cacheline >> + * of the lock. If that fails, check to see if it is the last node >> + * and so should clear the wait queue. >> + */ >> + next = ACCESS_ONCE(node->next); >> + if (likely(next)) >> + goto notify_next; >> + >> + /* >> + * Clear the wait queue if it is the last node >> + */ >> + if ((ACCESS_ONCE(lock->waitq) == node)&& >> + (cmpxchg(&lock->waitq, node, NULL) == node)) >> + return; >> + /* >> + * Wait until the next one in queue set up the next field >> + */ >> + while (likely(!(next = ACCESS_ONCE(node->next)))) >> + cpu_relax(); >> + /* >> + * The next one in queue is now at the head >> + */ >> +notify_next: >> + barrier(); >> + ACCESS_ONCE(next->wait) = false; >> + smp_wmb(); >> +} > I believe this could be unified with mspin_lock() / mspin_unlock() in > kernel/mutex.c ? (there is already talk of extending these functions > to be used by rwsem for adaptive spinning as well...)
It probably can, but the unification can wait until the code are in.
> Not a full review yet - I like the idea of making rwlock more fair but > I haven't dug too much into the details yet. >
Thank for taking the time to review it.
-Longman
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