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SubjectRe: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg
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Omar?


On 6/29/19 9:42 AM, Pavel Begunkov wrote:
> Ping?
>
> On 23/05/2019 08:39, Pavel Begunkov (Silence) wrote:
>> From: Pavel Begunkov <asml.silence@gmail.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@gmail.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);
>>
>> /*
>> * 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);
>> return ws;
>> }
>>
>>
>


--
Jens Axboe

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