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.
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