Re: [PATCH v2 2/4] sbitmap: remove swap_lock

[Ming Lei]

On Sun, Nov 22, 2020 at 03:35:46PM +0000, Pavel Begunkov wrote:
> map->swap_lock protects map->cleared from concurrent modification,
> however sbitmap_deferred_clear() is already atomically drains it, so
> it's guaranteed to not loose bits on concurrent
> sbitmap_deferred_clear().
> 
> A one threaded tag heavy test on top of nullbk showed ~1.5% t-put
> increase, and 3% -> 1% cycle reduction of sbitmap_get() according to perf.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> ---
>  include/linux/sbitmap.h |  5 -----
>  lib/sbitmap.c           | 14 +++-----------
>  2 files changed, 3 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index e40d019c3d9d..74cc6384715e 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -32,11 +32,6 @@ struct sbitmap_word {
>  	 * @cleared: word holding cleared bits
>  	 */
>  	unsigned long cleared ____cacheline_aligned_in_smp;
> -
> -	/**
> -	 * @swap_lock: Held while swapping word <-> cleared
> -	 */
> -	spinlock_t swap_lock;
>  } ____cacheline_aligned_in_smp;
>  
>  /**
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index c1c8a4e69325..4fd877048ba8 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -15,13 +15,9 @@
>  static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>  {
>  	unsigned long mask, val;
> -	bool ret = false;
> -	unsigned long flags;
>  
> -	spin_lock_irqsave(&map->swap_lock, flags);
> -
> -	if (!map->cleared)
> -		goto out_unlock;
> +	if (!READ_ONCE(map->cleared))
> +		return false;

This way might break sbitmap_find_bit_in_index()/sbitmap_get_shallow().
Currently if sbitmap_deferred_clear() returns false, it means nothing
can be allocated from this word. With this patch, even though 'false'
is returned, free bits still might be available because another
sbitmap_deferred_clear() can be run concurrently.

Thanks,
Ming