[PATCH] Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 05, 2021 at 04:00:01PM +0200, Peter Zijlstra wrote:

> No, when try_cmpxchg() fails it will update oldp. This is the reason old
> is now a pointer too.

Since you're not the first person confused by this, does the below
clarify?

---
Subject: Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon Jul  5 17:00:24 CEST 2021

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>
---
 Documentation/atomic_t.txt |   41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -271,3 +271,44 @@ because it would not order the W part of
 			SC *y, t;
 
 is allowed.
+
+
+CMPXHG 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.