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SubjectRe: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff
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Hi,

On 10/18/2022 4:44 PM, Hillf Danton wrote:
> On 17 Oct 2022 17:13:55 -0400 Waiman Long <longman@redhat.com>
>> @@ -1067,13 +1119,33 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
>> return sem;
>> }
>> adjustment += RWSEM_FLAG_WAITERS;
>> + } else if ((count & RWSEM_FLAG_HANDOFF) &&
>> + ((count & RWSEM_LOCK_MASK) == RWSEM_READER_BIAS)) {
>
> Could a couple of CPUs go read slow path in parallel?
>
>> + /*
>> + * If the waiter to be handed off is a reader, this reader
>> + * can piggyback on top of top of that.
>> + */
>> + if (rwsem_first_waiter(sem)->type == RWSEM_WAITING_FOR_READ)
>> + adjustment = 0;
>> + rwsem_handoff(sem, adjustment, &wake_q);
>> +
>> + if (!adjustment) {
>> + raw_spin_unlock_irq(&sem->wait_lock);
>> + wake_up_q(&wake_q);
>> + return sem;
>> + }
>> + adjustment = 0;
>> }
>> rwsem_add_waiter(sem, &waiter);
>
> Why can this acquirer pigyback without becoming a waiter?
>
>>
>> - /* we're now waiting on the lock, but no longer actively locking */
>> - count = atomic_long_add_return(adjustment, &sem->count);
>> -
>> - rwsem_cond_wake_waiter(sem, count, &wake_q);
>> + if (adjustment) {
>> + /*
>> + * We are now waiting on the lock with no handoff, but no
>> + * longer actively locking.
>> + */
>> + count = atomic_long_add_return(adjustment, &sem->count);
>> + rwsem_cond_wake_waiter(sem, count, &wake_q);
>> + }
>> raw_spin_unlock_irq(&sem->wait_lock);
>>
>> if (!wake_q_empty(&wake_q))
>> @@ -1120,7 +1192,6 @@ static struct rw_semaphore __sched *
>> rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>> {
>> struct rwsem_waiter waiter;
>> - int null_owner_retries;
>
> This reverts 2/5 and feel free to drop it directly.

I think, he intents to tag the first two patches to go to stable branches.

-Mukesh
>
> Hillf

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