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SubjectRe: [PATCH v2 2/4] sbitmap: remove swap_lock
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On 22/11/2020 15:35, 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@gmail.com>
> ---
> 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;

So if we race with another cpu, won't the 2nd cpu see that the mask is 0
returned from the xchg (not shown)? If so, it's odd to continue to do
the CAS - or atomic not, from later patch - on a mask of 0.

Thanks,
John

>
> /*
> * First get a stable cleared mask, setting the old mask to 0.
> @@ -35,10 +31,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
> val = map->word;
> } while (cmpxchg(&map->word, val, val & ~mask) != val);
>
> - ret = true;
> -out_unlock:
> - spin_unlock_irqrestore(&map->swap_lock, flags);
> - return ret;
> + return true;
> }
>
> int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
> @@ -80,7 +73,6 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
> for (i = 0; i < sb->map_nr; i++) {
> sb->map[i].depth = min(depth, bits_per_word);
> depth -= sb->map[i].depth;
> - spin_lock_init(&sb->map[i].swap_lock);
> }
> return 0;
> }
>

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