[tip: locking/core] Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     d1bbfd0c7c9f985e57795a7e0cefc209ebf689c0
Gitweb:        https://git.kernel.org/tip/d1bbfd0c7c9f985e57795a7e0cefc209ebf689c0
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 05 Jul 2021 17:00:24 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 07 Jul 2021 13:53:25 +02:00

Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()

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@xxxxxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/YOMgPeMOmmiK3tXO@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 Documentation/atomic_t.txt | 41 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+)

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index 0f1fded..a9c1e2b 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.