linux-kernel - Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

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Date:   Mon, 1 Jul 2019 10:35:34 -0700
From:   Omar Sandoval <osandov@...ndov.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, osandov@...com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

On Sat, Jun 29, 2019 at 08:42:23AM -0700, Pavel Begunkov wrote:
> Ping?
> 
> On 23/05/2019 08:39, Pavel Begunkov (Silence) wrote:
> > From: Pavel Begunkov <asml.silence@...il.com>
> > 
> > cmpxchg() with an immediate value could be replaced with less expensive
> > xchg(). The same true if new value don't _depend_ on the old one.
> > 
> > In the second block, atomic_cmpxchg() return value isn't checked, so
> > after atomic_cmpxchg() ->  atomic_xchg() conversion it could be replaced
> > with atomic_set(). Comparison with atomic_read() in the second chunk was
> > left as an optimisation (if that was the initial intention).
> > 
> > Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> > ---
> >  lib/sbitmap.c | 10 +++-------
> >  1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index 155fe38756ec..7d7e0e278523 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -37,9 +37,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
> >  	/*
> >  	 * First get a stable cleared mask, setting the old mask to 0.
> >  	 */
> > -	do {
> > -		mask = sb->map[index].cleared;
> > -	} while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask);
> > +	mask = xchg(&sb->map[index].cleared, 0);

This hunk is clearly correct.

> >  	/*
> >  	 * Now clear the masked bits in our free word
> > @@ -527,10 +525,8 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> >  		struct sbq_wait_state *ws = &sbq->ws[wake_index];
> >  
> >  		if (waitqueue_active(&ws->wait)) {
> > -			int o = atomic_read(&sbq->wake_index);
> > -
> > -			if (wake_index != o)
> > -				atomic_cmpxchg(&sbq->wake_index, o, wake_index);
> > +			if (wake_index != atomic_read(&sbq->wake_index))
> > +				atomic_set(&sbq->wake_index, wake_index);

This hunk used to imply a memory barrier and no longer does. I don't
think that's a problem, though.

Reviewed-by: Omar Sandoval <osandov@...com>