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SubjectRe: [PATCH v4 1/3] qrwlock: A queue read/write lock implementation
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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