Generally, both atomic_inc_unless_negative() and
atomic_dec_unless_positive() need at least two atomic_cmpxchg()
to complete the atomic operation. In fact, the 1st atomic_cmpxchg()
is just used to read current value of the atomic variable at most times.
Considered memory barrier, bus lock, cache walking, etc. things may be
involved in atomic_cmpxchg(), it is much expensive than atomic_read(),
which is just the simple below:
(*(volatile int *)&(v)->counter)
so this patch can save one extra atomic_cmpxchg() for the two
helpers under general situation, and should improve them a bit.
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Shaohua Li <s...@kernel.org>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Signed-off-by: Ming Lei <tom.leim...@gmail.com>
---
include/linux/atomic.h | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 5b08a85..aa951d8 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -63,26 +63,34 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int
hint)
#ifndef atomic_inc_unless_negative
static inline int atomic_inc_unless_negative(atomic_t *p)
{
- int v, v1;
- for (v = 0; v >= 0; v = v1) {
- v1 = atomic_cmpxchg(p, v, v + 1);
- if (likely(v1 == v))
+ int v, t;
+
+ v = atomic_read(p);
+ while (1) {
+ if (unlikely(v < 0))
+ return 0;
+ t = atomic_cmpxchg(p, v, v + 1);
+ if (likely(t == v))
return 1;
+ v = t;
}
- return 0;
}
#endif
#ifndef atomic_dec_unless_positive
static inline int atomic_dec_unless_positive(atomic_t *p)
{
- int v, v1;
- for (v = 0; v <= 0; v = v1) {
- v1 = atomic_cmpxchg(p, v, v - 1);
- if (likely(v1 == v))
+ int v, t;
+
+ v = atomic_read(p);
+ while (1) {
+ if (unlikely(v > 0))
+ return 0;
+ t = atomic_cmpxchg(p, v, v - 1);
+ if (likely(t == v))
return 1;
+ v = t;
}
- return 0;
}
#endif
--
1.7.9.5
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