[PATCH] bcache: optimize barrier usage for Rmw atomic bitops

[PATCH] bcache: optimize barrier usage for Rmw atomic bitops

[Davidlohr Bueso]

We can avoid the unnecessary barrier on non LL/SC architectures,
such as x86. Instead, use the smp_mb__after_atomic().

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 drivers/md/bcache/writeback.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 4a40f9eadeaf..284ff10f480f 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -183,7 +183,7 @@ static void update_writeback_rate(struct work_struct *work)
 	 */
 	set_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
-	smp_mb();
+	smp_mb__after_atomic();
 
 	/*
 	 * CACHE_SET_IO_DISABLE might be set via sysfs interface,
@@ -193,7 +193,7 @@ static void update_writeback_rate(struct work_struct *work)
 	    test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
 		clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 		/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
-		smp_mb();
+		smp_mb__after_atomic();
 		return;
 	}
 
@@ -229,7 +229,7 @@ static void update_writeback_rate(struct work_struct *work)
 	 */
 	clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
 	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
-	smp_mb();
+	smp_mb__after_atomic();
 }
 
 static unsigned int writeback_delay(struct cached_dev *dc,
-- 
2.16.4

Re: [PATCH] bcache: optimize barrier usage for Rmw atomic bitops

[Coly Li]

On 2020/1/30 2:08 上午, Davidlohr Bueso wrote:
> We can avoid the unnecessary barrier on non LL/SC architectures,
> such as x86. Instead, use the smp_mb__after_atomic().
> 

Hi Davidlohr,

> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

Sorry for replying late. Indeed we don't need to worry about the extra
smp_mb() because the code patch gets called every 5 seconds by default,
which is very infrequent. But I like this more accurate patch, it is in
my testing already.

And by your inspiration, I also fix some other locations to use
smp_mb__before/after_atomic().

Thank you.

Coly Li

> ---
>  drivers/md/bcache/writeback.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 4a40f9eadeaf..284ff10f480f 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -183,7 +183,7 @@ static void update_writeback_rate(struct work_struct *work)
>  	 */
>  	set_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>  	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
> -	smp_mb();
> +	smp_mb__after_atomic();
>  
>  	/*
>  	 * CACHE_SET_IO_DISABLE might be set via sysfs interface,
> @@ -193,7 +193,7 @@ static void update_writeback_rate(struct work_struct *work)
>  	    test_bit(CACHE_SET_IO_DISABLE, &c->flags)) {
>  		clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>  		/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
> -		smp_mb();
> +		smp_mb__after_atomic();
>  		return;
>  	}
>  
> @@ -229,7 +229,7 @@ static void update_writeback_rate(struct work_struct *work)
>  	 */
>  	clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
>  	/* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
> -	smp_mb();
> +	smp_mb__after_atomic();
>  }
>  
>  static unsigned int writeback_delay(struct cached_dev *dc,
> 


-- 

Coly Li