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authorPeter Zijlstra <peterz@infradead.org>2021-07-05 18:00:24 +0300
committerPeter Zijlstra <peterz@infradead.org>2021-07-07 14:53:25 +0300
commitd1bbfd0c7c9f985e57795a7e0cefc209ebf689c0 (patch)
tree58385a9487e3650dbadc7acf305e6dbe60d7b298 /Documentation/atomic_t.txt
parente6b4457b05f36bb9e371f29ab1dd2d97272a1540 (diff)
downloadlinux-d1bbfd0c7c9f985e57795a7e0cefc209ebf689c0.tar.xz
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@infradead.org> Acked-by: Will Deacon <will@kernel.org> Link: https://lkml.kernel.org/r/YOMgPeMOmmiK3tXO@hirez.programming.kicks-ass.net
Diffstat (limited to 'Documentation/atomic_t.txt')
-rw-r--r--Documentation/atomic_t.txt41
1 files changed, 41 insertions, 0 deletions
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.