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Subject[PATCH v2 2/4] sbitmap: remove swap_lock
Date
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;

/*
* 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;
}
--
2.24.0
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