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SubjectRe: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer
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On 04/17/2019 09:56 AM, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
>> @@ -549,7 +582,7 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
>> return !owner ? OWNER_NULL : OWNER_READER;
>> }
>>
>> -static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> +static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
>> {
>> bool taken = false;
>> bool is_rt_task = rt_task(current);
>> @@ -558,9 +591,6 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> preempt_disable();
>>
>> /* sem->wait_lock should not be held when doing optimistic spinning */
>> - if (!rwsem_can_spin_on_owner(sem))
>> - goto done;
>> -
>> if (!osq_lock(&sem->osq))
>> goto done;
>>
>> @@ -580,10 +610,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>> /*
>> * Try to acquire the lock
>> */
>> - if (rwsem_try_write_lock_unqueued(sem)) {
>> - taken = true;
>> + taken = wlock ? rwsem_try_write_lock_unqueued(sem)
>> + : rwsem_try_read_lock_unqueued(sem);
>> +
>> + if (taken)
>> break;
>> - }
>>
>> /*
>> * An RT task cannot do optimistic spinning if it cannot
> Alternatively you pass the trylock function as an argument:
>
> static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
> bool (*trylock)(struct rw_semaphore *sem))
> {
> ...
> if (trylock(sem)) {
> taken = true;
> goto unlock;
> }
> ...
> }
>
With retpoline, an indirect function call will be slower.

Cheers,
Longman


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