Re: [PATCHv2] sbitmap: fix batched wait_cnt accounting

[Pankaj Raghav <pankydev8@gmail.com>]

On Thu, Aug 25, 2022 at 01:48:43PM +0200, Pankaj Raghav wrote:
> On Wed, Aug 24, 2022 at 01:14:40PM -0700, Keith Busch wrote:
> >  
> > -static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> > +static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
> >  {
> >  	struct sbq_wait_state *ws;
> > -	unsigned int wake_batch;
> > -	int wait_cnt;
> > +	int wake_batch, wait_cnt, sub, cur;
> >  
> >  	ws = sbq_wake_ptr(sbq);
> >  	if (!ws)
> >  		return false;
> >  
> > -	wait_cnt = atomic_dec_return(&ws->wait_cnt);
> > +	wake_batch = READ_ONCE(sbq->wake_batch);
> > +	do {
> > +		cur = atomic_read(&ws->wait_cnt);
> 
> I think the above statement is not needed if we use atomic_try_cmpxchg
> as the old value is updated in that function itself.
> https://docs.kernel.org/staging/index.html?highlight=atomic_try_cmpxchg#atomic-types

I mean after moving the cur = atomic_read(..) above the do statement:

wake_batch = READ_ONCE(sbq->wake_batch);
cur = atomic_read(&ws->wait_cnt);
do {
	sub = min3(wake_batch, *nr, cur);
	wait_cnt = cur - sub;
} while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt));

-- 
Pankaj Raghav