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SubjectRe: [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on_owner() return owner state
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On 04/17/2019 08:47 AM, Peter Zijlstra wrote:
> On Wed, Apr 17, 2019 at 02:41:01PM +0200, Peter Zijlstra wrote:
>> On Sat, Apr 13, 2019 at 01:22:51PM -0400, Waiman Long wrote:
>>> In the special case that there is no active lock and the handoff bit
>>> is set, optimistic spinning has to be stopped.
>>> @@ -500,9 +521,19 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
>>>
>>> /*
>>> * If there is a new owner or the owner is not set, we continue
>>> - * spinning.
>>> + * spinning except when here is no active locks and the handoff bit
>>> + * is set. In this case, we have to stop spinning.
>>> */
>>> - return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
>>> + owner = READ_ONCE(sem->owner);
>>> + if (!is_rwsem_owner_spinnable(owner))
>>> + return OWNER_NONSPINNABLE;
>>> + if (owner && !is_rwsem_owner_reader(owner))
>>> + return OWNER_WRITER;
>>> +
>>> + count = atomic_long_read(&sem->count);
>>> + if (RWSEM_COUNT_HANDOFF(count) && !RWSEM_COUNT_LOCKED(count))
>>> + return OWNER_NONSPINNABLE;
>>> + return !owner ? OWNER_NULL : OWNER_READER;
>>> }
>> So this fixes a straight up bug in the last patch (and thus should be
>> done before so the bug never exists), and creates unreadable code while
>> at it.
>>
>> Also, I think only checking HANDOFF after the loop is wrong; the moment
>> HANDOFF happens you have to terminate the loop, irrespective of what
>> @owner does.
>>
>> Does something like so work?
>>
>> ---
>>
>> enum owner_state {
>> OWNER_NULL = 1 << 0,
>> OWNER_WRITER = 1 << 1,
>> OWNER_READER = 1 << 2,
>> OWNER_NONSPINNABLE = 1 << 3,
>> };
>> #define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER)
> Hmm, we should not spin on OWNER_NULL. Or at least not mixed in with the
> patch that changes the shape of all this. That should go in the RT
> thingy patch, which comes after this.

We do spin on OWNER_NULL right now, not in rwsem_spin_on_owner() but in
the main rwsem_optimistic_spin() function.

RT task will quit if owner is NULL.

Cheers,
Longman

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