Re: sbitmap: check cleared bits when iterating busy bits

[Omar Sandoval <osandov@osandov.com>]

On Mon, Dec 03, 2018 at 02:56:17PM -0700, Jens Axboe wrote:
> When we are iterating the set bits in a word, we also need to factor in
> the cleared bits. Don't call fn() unless the bit is also not set in
> the cleared word.
> 
> Fixes: ea86ea2cdced ("sbitmap: ammortize cost of clearing bits")
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index 92806a2dbab7..9f374fbcdba6 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -283,6 +283,11 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
>  			nr = find_next_bit(&word->word, depth, nr);
>  			if (nr >= depth)
>  				break;
> +			/* if set in cleared, it's actually free */
> +			if (test_bit(nr, &word->cleared)) {
> +				nr++;
> +				continue;
> +			}
>  			if (!fn(sb, (index << sb->shift) + nr, data))
>  				return;
>  
> -- 
> Jens Axboe
> 

How about something like this:

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index f0f49bbb2617..fe9122386255 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -265,12 +265,14 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
 	nr = SB_NR_TO_BIT(sb, start);
 
 	while (scanned < sb->depth) {
-		struct sbitmap_word *word = &sb->map[index];
-		unsigned int depth = min_t(unsigned int, word->depth - nr,
+		unsigned long word;
+		unsigned int depth = min_t(unsigned int,
+					   sb->map[index].depth - nr,
 					   sb->depth - scanned);
 
 		scanned += depth;
-		if (!word->word)
+		word = sb->map[index].word & ~sb->map[index].cleared;
+		if (!word)
 			goto next;
 
 		/*
@@ -280,7 +282,7 @@ static inline void __sbitmap_for_each_set(struct sbitmap *sb,
 		 */
 		depth += nr;
 		while (1) {
-			nr = find_next_bit(&word->word, depth, nr);
+			nr = find_next_bit(&word, depth, nr);
 			if (nr >= depth)
 				break;
 			if (!fn(sb, (index << sb->shift) + nr, data))

Might be marginally faster.