Messages in this thread | | | Subject | Re: [PATCH v2 5/5] locking/rwsem: Remove reader optimistic spinning | From | Waiman Long <> | Date | Tue, 8 Dec 2020 10:29:39 -0500 |
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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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