The following commit has been merged into the locking/core branch of tip:
Commit-ID: e807c2a37044a51de89d6d4f8a1f5ecfb3752f36
Gitweb: https://git.kernel.org/tip/e807c2a37044a51de89d6d4f8a1f5ecfb3752f36
Author: Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate: Wed, 24 Jan 2024 11:58:16 +01:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 01 Mar 2024 12:54:25 +01:00
locking/x86: Implement local_xchg() using CMPXCHG without the LOCK prefix
Implement local_xchg() using the CMPXCHG instruction without the LOCK prefix.
XCHG is expensive due to the implied LOCK prefix. The processor
cannot prefetch cachelines if XCHG is used.
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Link: https://lore.kernel.org/r/20240124105816.612670-1-ubizjak@xxxxxxxxx
---
arch/x86/include/asm/local.h | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
index 73dba8b..59aa966 100644
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -131,8 +131,20 @@ static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
(typeof(l->a.counter) *) old, new);
}
-/* Always has a lock prefix */
-#define local_xchg(l, n) (xchg(&((l)->a.counter), (n)))
+/*
+ * Implement local_xchg using CMPXCHG instruction without the LOCK prefix.
+ * XCHG is expensive due to the implied LOCK prefix. The processor
+ * cannot prefetch cachelines if XCHG is used.
+ */
+static __always_inline long
+local_xchg(local_t *l, long n)
+{
+ long c = local_read(l);
+
+ do { } while (!local_try_cmpxchg(l, &c, n));
+
+ return c;
+}
/**
* local_add_unless - add unless the number is already a given value
|