On 2023/7/21 03:06, Gabriel Krisman Bertazi wrote:
> chengming.zhou@xxxxxxxxx writes:
>
>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>
>> ```
>> 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@xxxxxxxxxxxxx>
>> ---
>> 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;
>
