[PATCH 1/1] sbitmap: Replace cmpxchg with xchg

[PATCH 1/1] sbitmap: Replace cmpxchg with xchg

[Pavel Begunkov (Silence)]

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;
 		}
 
-- 
2.21.0

Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

[Pavel Begunkov]

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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;
>  		}
>  
> 

-- 
Yours sincerely,
Pavel Begunkov


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Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

[Jens Axboe]

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

Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

[Omar Sandoval]

On Sat, Jun 29, 2019 at 08:42:23AM -0700, 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);

This hunk is clearly correct.

> >  	/*
> >  	 * 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);

This hunk used to imply a memory barrier and no longer does. I don't
think that's a problem, though.

Reviewed-by: Omar Sandoval <osandov@fb.com>

Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

[Jens Axboe]

On 5/23/19 9:39 AM, 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).

Applied, thanks.

-- 
Jens Axboe