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SubjectRe: [PATCH v2 5/5] locking/rwsem: Remove reader optimistic spinning
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On 12/8/20 5:07 AM, Peter Zijlstra wrote:
> On Fri, Nov 20, 2020 at 11:14:16PM -0500, Waiman Long wrote:
>
>
>> @@ -1032,40 +901,16 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count)
>> *
>> * We can take the read lock directly without doing
>> * rwsem_optimistic_spin() if the conditions are right.
> This comment no longer makes sense..

You are right. I forgot to take that out.


>> - * Also wake up other readers if it is the first reader.
>> */
>> - if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF)) &&
>> - rwsem_no_spinners(sem)) {
>> + if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
>> rwsem_set_reader_owned(sem);
>> lockevent_inc(rwsem_rlock_steal);
>> - if (rcnt == 1)
>> - goto wake_readers;
>> - return sem;
>> - }
>>
>> - /*
>> - * Save the current read-owner of rwsem, if available, and the
>> - * reader nonspinnable bit.
>> - */
>> - waiter.last_rowner = owner;
>> - if (!(waiter.last_rowner & RWSEM_READER_OWNED))
>> - waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
>> -
>> - if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
>> - goto queue;
>> -
>> - /*
>> - * Undo read bias from down_read() and do optimistic spinning.
>> - */
>> - atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
>> - adjustment = 0;
>> - if (rwsem_optimistic_spin(sem, false)) {
> since we're removing the optimistic spinning entirely on the read side.
>
> Also, I was looking at skipping patch #4, which mucks with the reader
> wakeup logic, and afaict this removal doesn't really depend on it.
>
> Or am I missing something?

That is true. Patch 4 isn't essential for this series. So if you are
general OK with the current patchset, I can send out v3 that remove
patch 4 and make the your suggested change above.

Cheers,
Longman

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