sbitmap: remove swap_lock

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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index e40d019..74cc638 100644 (file)
--- 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 c1c8a4e..4fd8770 100644 (file)
--- 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;
 }