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.sile...@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;
  }



Reply via email to