From d1bbfd0c7c9f985e57795a7e0cefc209ebf689c0 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon, 5 Jul 2021 17:00:24 +0200
Subject: [PATCH] Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()
Git-commit: d1bbfd0c7c9f985e57795a7e0cefc209ebf689c0
Patch-mainline: v5.15-rc1
References: bsc#1190137
There seems to be a significant amount of confusion around the new
try_cmpxchg(), despite this being more like the C11
atomic_compare_exchange_*() family. Add a few words of clarification
on how cmpxchg() and try_cmpxchg() relate to one another.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/YOMgPeMOmmiK3tXO@hirez.programming.kicks-ass.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
Documentation/atomic_t.txt | 41 ++++++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)
diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index 0f1fdedf36bb..a9c1e2b39b15 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -271,3 +271,44 @@ WRITE_ONCE. Thus:
SC *y, t;
is allowed.
+
+
+CMPXCHG vs TRY_CMPXCHG
+----------------------
+
+ int atomic_cmpxchg(atomic_t *ptr, int old, int new);
+ bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new);
+
+Both provide the same functionality, but try_cmpxchg() can lead to more
+compact code. The functions relate like:
+
+ bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new)
+ {
+ int ret, old = *oldp;
+ ret = atomic_cmpxchg(ptr, old, new);
+ if (ret != old)
+ *oldp = ret;
+ return ret == old;
+ }
+
+and:
+
+ int atomic_cmpxchg(atomic_t *ptr, int old, int new)
+ {
+ (void)atomic_try_cmpxchg(ptr, &old, new);
+ return old;
+ }
+
+Usage:
+
+ old = atomic_read(&v); old = atomic_read(&v);
+ for (;;) { do {
+ new = func(old); new = func(old);
+ tmp = atomic_cmpxchg(&v, old, new); } while (!atomic_try_cmpxchg(&v, &old, new));
+ if (tmp == old)
+ break;
+ old = tmp;
+ }
+
+NB. try_cmpxchg() also generates better code on some platforms (notably x86)
+where the function more closely matches the hardware instruction.
--
2.26.2