From: Chengming Zhou <zhouchengming@bytedance.com>

```
hint = nr + 1;
if (hint >= depth - 1)
	hint = 0;
```

Now we wrap the hint to 0 in the failure case, but:
1. hint == depth - 1, is actually an available offset hint, which
   we shouldn't wrap hint to 0.
2. In the strict round_robin non-wrap case, we shouldn't wrap at all.

```
wrap = wrap && hint;
```

We only need to check wrap based on the original hint ( > 0), don't need
to recheck the new hint which maybe updated in the failure case.
Also delete the mismatched comments by the way.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 lib/sbitmap.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index eff4e42c425a..5ed6c2adf58e 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -144,12 +144,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
 	while (1) {
 		nr = find_next_zero_bit(word, depth, hint);
 		if (unlikely(nr >= depth)) {
-			/*
-			 * We started with an offset, and we didn't reset the
-			 * offset to 0 in a failure case, so start from 0 to
-			 * exhaust the map.
-			 */
-			if (hint && wrap) {
+			if (wrap) {
 				hint = 0;
 				continue;
 			}
@@ -160,8 +155,13 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
 			break;
 
 		hint = nr + 1;
-		if (hint >= depth - 1)
-			hint = 0;
+		if (hint >= depth) {
+			if (wrap) {
+				hint = 0;
+				continue;
+			}
+			return -1;
+		}
 	}
 
 	return nr;
-- 
2.41.0

chengming.zhou@linux.dev writes:

> From: Chengming Zhou <zhouchengming@bytedance.com>
>
> ```
> hint = nr + 1;
> if (hint >= depth - 1)
> 	hint = 0;
> ```
>
> Now we wrap the hint to 0 in the failure case, but:
> 1. hint == depth - 1, is actually an available offset hint, which
>    we shouldn't wrap hint to 0.
> 2. In the strict round_robin non-wrap case, we shouldn't wrap at all.
>
> ```
> wrap = wrap && hint;
> ```
>
> We only need to check wrap based on the original hint ( > 0), don't need
> to recheck the new hint which maybe updated in the failure case.
> Also delete the mismatched comments by the way.
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
>  lib/sbitmap.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index eff4e42c425a..5ed6c2adf58e 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -144,12 +144,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>  	while (1) {
>  		nr = find_next_zero_bit(word, depth, hint);
>  		if (unlikely(nr >= depth)) {
> -			/*
> -			 * We started with an offset, and we didn't reset the
> -			 * offset to 0 in a failure case, so start from 0 to
> -			 * exhaust the map.
> -			 */
> -			if (hint && wrap) {
> +			if (wrap) {
>  				hint = 0;
>  				continue;

I think this is wrong.  If you start with an offset in the wrap case and
the bitmap is completely full this will become busy wait until a bit is
available. The hint check is what make you break out of the loop early,
after wrapping, re-walking the entire bitmap and failing to find any
available space.

> @@ -160,8 +155,13 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>  			break;
>  
>  		hint = nr + 1;
> -		if (hint >= depth - 1)
> -			hint = 0;
> +		if (hint >= depth) {
> +			if (wrap) {
> +				hint = 0;
> +				continue;
> +			}
> +			return -1;
> +		}
>  	}
>  
>  	return nr;

-- 
Gabriel Krisman Bertazi

On 2023/7/21 03:06, Gabriel Krisman Bertazi wrote:
> chengming.zhou@linux.dev writes:
> 
>> From: Chengming Zhou <zhouchengming@bytedance.com>
>>
>> ```
>> hint = nr + 1;
>> if (hint >= depth - 1)
>> 	hint = 0;
>> ```
>>
>> Now we wrap the hint to 0 in the failure case, but:
>> 1. hint == depth - 1, is actually an available offset hint, which
>>    we shouldn't wrap hint to 0.
>> 2. In the strict round_robin non-wrap case, we shouldn't wrap at all.
>>
>> ```
>> wrap = wrap && hint;
>> ```
>>
>> We only need to check wrap based on the original hint ( > 0), don't need
>> to recheck the new hint which maybe updated in the failure case.
>> Also delete the mismatched comments by the way.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> ---
>>  lib/sbitmap.c | 16 ++++++++--------
>>  1 file changed, 8 insertions(+), 8 deletions(-)
>>
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index eff4e42c425a..5ed6c2adf58e 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -144,12 +144,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>>  	while (1) {
>>  		nr = find_next_zero_bit(word, depth, hint);
>>  		if (unlikely(nr >= depth)) {
>> -			/*
>> -			 * We started with an offset, and we didn't reset the
>> -			 * offset to 0 in a failure case, so start from 0 to
>> -			 * exhaust the map.
>> -			 */
>> -			if (hint && wrap) {
>> +			if (wrap) {
>>  				hint = 0;
>>  				continue;
> 
> I think this is wrong.  If you start with an offset in the wrap case and
> the bitmap is completely full this will become busy wait until a bit is
> available. The hint check is what make you break out of the loop early,
> after wrapping, re-walking the entire bitmap and failing to find any
> available space.

Ah yes, you are right, thanks for your explanation. Here we need to check
"hint && wrap" to avoid wrap repeatedly.

Will drop this change in the next version.

> 
>> @@ -160,8 +155,13 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>>  			break;
>>  
>>  		hint = nr + 1;

Here we overwrite hint, may cause repeated wrap. So I think it's clearer that
we set "wrap" to false after we wrap?

```
if (wrap) {
	wrap = false;
	hint = 0;
	continue;
}
```

Thanks!

>> -		if (hint >= depth - 1)
>> -			hint = 0;
>> +		if (hint >= depth) {
>> +			if (wrap) {
>> +				hint = 0;
>> +				continue;
>> +			}
>> +			return -1;
>> +		}
>>  	}
>>  
>>  	return nr;
>