Re: [PATCH v2 5/5] locking/rwsem: Remove reader optimistic spinning

[Waiman Long]

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