linux-kernel - Re: [PATCH RESEND v2 2/5] sbitmap: remove redundant check in __sbitmap_queue_get

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Message-ID: <20221222121610.6o23vbarpiaqwkcx@quack3>
Date:   Thu, 22 Dec 2022 13:16:10 +0100
From:   Jan Kara <jack@...e.cz>
To:     Kemeng Shi <shikemeng@...weicloud.com>
Cc:     Jan Kara <jack@...e.cz>, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        kbusch@...nel.org, mwilck@...e.com, wuchi <wuchi.zero@...il.com>
Subject: Re: [PATCH RESEND v2 2/5] sbitmap: remove redundant check in
 __sbitmap_queue_get_batch

[Added to CC original author of the problematic commit and reviewer]

On Thu 22-12-22 19:49:12, Kemeng Shi wrote:
> Hi Jan, thanks for review.
> on 12/22/2022 7:23 PM, Jan Kara wrote:
> >> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> >> index cb5e03a2d65b..11e75f4040fb 100644
> >> --- a/lib/sbitmap.c
> >> +++ b/lib/sbitmap.c
> >> @@ -518,11 +518,9 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
> >>  
> >>  			get_mask = ((1UL << nr_tags) - 1) << nr;
> >>  			val = READ_ONCE(map->word);
> >> -			do {
> >> -				if ((val & ~get_mask) != val)
> >> -					goto next;
> >> -			} while (!atomic_long_try_cmpxchg(ptr, &val,
> >> -							  get_mask | val));
> >> +			while (!atomic_long_try_cmpxchg(ptr, &val,
> >> +							  get_mask | val))
> >> +				;
> >>  			get_mask = (get_mask & ~val) >> nr;
> >>  			if (get_mask) {
> >>  				*offset = nr + (index << sb->shift);
> > 
> > So I agree this will result in correct behavior but it can change
> > performance. In the original code, we end up doing
> > atomic_long_try_cmpxchg() only for words where we have a chance of getting
> > all tags allocated. Now we just accept any word where we could allocate at
> > least one bit. Frankly the original code looks rather restrictive and also
> > the fact that we look only from the first zero bit in the word looks
> > unnecessarily restrictive so maybe I miss some details about what's
> > expected from __sbitmap_queue_get_batch(). So all in all I wanted to point
> > out this needs more scrutiny from someone understanding better expectations
> > from __sbitmap_queue_get_batch().
> In the very beginning, __sbitmap_queue_get_batch will return if we only
> get partial tags allocated. Recent commit fbb564a557809 ("lib/sbitmap: Fix
> invalid loop in __sbitmap_queue_get_batch()") thought we may reuse busying
> bits in old codes and change behavior of __sbitmap_queue_get_batch() to get
> all tags. However we will not reuse busying bits in old codes actually. So
> I try to revert this wrong fix and keep the behavior of
> __sbitmap_queue_get_batch() as it designed to be at beginning.

I see and now I agree. Please add tag:

Fixes: fbb564a557809 ("lib/sbitmap: Fix invalid loop in __sbitmap_queue_get_batch()") 

to your commit. Also feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>
 
> Besides, if we keep to get all tags,the check below is redundant.
> 	get_mask = (get_mask & ~ret) >> nr;
> 	if (get_mask) {
> 		...
> 	}
> As we only reach here if we get all tags and the check above will always
> pass. So this check in old codes should be removed.

Yeah, I agree.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR