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Subject: Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg
To:     Pavel Begunkov <asml.silence@gmail.com>, osandov@fb.com,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <722e1d561f0a49dc464d3a2b1be4c077f7502726.1558625912.git.asml.silence@gmail.com>
 <0979d23f-bc31-b653-51d0-867dd52bf7ee@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <7da51f2b-fda4-0aa4-717e-c13c80cced3e@kernel.dk>
Date:   Sat, 29 Jun 2019 09:51:26 -0600
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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