The patch titled
Subject: atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive
has been added to the -mm tree. Its filename is
atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive.patch
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From: Ming Lei <tom.leiming@xxxxxxxxx>
Subject: atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive
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.
Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/atomic.h | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
diff -puN include/linux/atomic.h~atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive include/linux/atomic.h
--- a/include/linux/atomic.h~atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive
+++ a/include/linux/atomic.h
@@ -63,26 +63,34 @@ static inline int atomic_inc_not_zero_hi
#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
_
Patches currently in -mm which might be from tom.leiming@xxxxxxxxx are
atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive.patch
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