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Subject[PATCH 2/2] ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below
Date
Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in atomic_long_inc_below.
x86 CMPXCHG instruction returns success in ZF flag, so this change saves
a compare after cmpxchg (and related move instruction in front of cmpxchg).

Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
---
kernel/ucount.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 974ac1585009..5d980460104e 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -218,16 +218,14 @@ void put_ucounts(struct ucounts *ucounts)

static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
{
- long c, old;
- c = atomic_long_read(v);
- for (;;) {
+ long c = atomic_long_read(v);
+
+ do {
if (unlikely(c >= u))
return false;
- old = atomic_long_cmpxchg(v, c, c+1);
- if (likely(old == c))
- return true;
- c = old;
- }
+ } while (!atomic_long_try_cmpxchg(v, &c, c+1));
+
+ return true;
}

struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
--
2.35.3
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