[PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks

Peter Zijlstra
2020-02-18T14:27:15Z

On Tue, Feb 18, 2020 at 02:13:10PM +0100, Peter Zijlstra wrote:
> (with the caveat that try_cmpxchg() doesn't seem available on !x86 -- I
> should go fix that)

Completely untested (lemme go do that shortly), but something like so I
suppose.

---
Subject: asm-generic/atomic: Add try_cmpxchg() fallbacks

Only x86 provides try_cmpxchg() outside of the atomic_t interfaces,
provide generic fallbacks to create this interface from the widely
available cmpxchg() function.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
diff --git a/include/linux/atomic-fallback.h b/include/linux/atomic-fallback.h
index 656b5489b673..243f61d6c35f 100644
--- a/include/linux/atomic-fallback.h
+++ b/include/linux/atomic-fallback.h
@@ -77,6 +77,50 @@

#endif /* cmpxchg64_relaxed */

+#ifndef try_cmpxchg
+#define try_cmpxchg(_ptr, _oldp, _new) \
+({ \
+ typeof(*ptr) ___r, ___o = *(_oldp); \
+ ___r = cmpxchg((_ptr), ___o, (_new)); \
+ if (unlikely(___r != ___o)) \
+ *(_old) = ___r; \
+ likely(___r == ___o); \
+})
+#endif /* try_cmpxchg */
+
+#ifndef try_cmpxchg_acquire
+#define try_cmpxchg_acquire(_ptr, _oldp, _new) \
+({ \
+ typeof(*ptr) ___r, ___o = *(_oldp); \
+ ___r = cmpxchg_acquire((_ptr), ___o, (_new)); \
+ if (unlikely(___r != ___o)) \
+ *(_old) = ___r; \
+ likely(___r == ___o); \
+})
+#endif /* try_cmpxchg_acquire */
+
+#ifndef try_cmpxchg_release
+#define try_cmpxchg_release(_ptr, _oldp, _new) \
+({ \
+ typeof(*ptr) ___r, ___o = *(_oldp); \
+ ___r = cmpxchg_release((_ptr), ___o, (_new)); \
+ if (unlikely(___r != ___o)) \
+ *(_old) = ___r; \
+ likely(___r == ___o); \
+})
+#endif /* try_cmpxchg_release */
+
+#ifndef try_cmpxchg_relaxed
+#define try_cmpxchg_relaxed(_ptr, _oldp, _new) \
+({ \
+ typeof(*ptr) ___r, ___o = *(_oldp); \
+ ___r = cmpxchg_relaxed((_ptr), ___o, (_new)); \
+ if (unlikely(___r != ___o)) \
+ *(_old) = ___r; \
+ likely(___r == ___o); \
+})
+#endif /* try_cmpxchg_relaxed */
+
#ifndef atomic_read_acquire
static __always_inline int
atomic_read_acquire(const atomic_t *v)
@@ -2294,4 +2338,4 @@ atomic64_dec_if_positive(atomic64_t *v)
#define atomic64_cond_read_relaxed(v, c) smp_cond_load_relaxed(&(v)->counter, (c))

#endif /* _LINUX_ATOMIC_FALLBACK_H */
-// baaf45f4c24ed88ceae58baca39d7fd80bb8101b
+// 2dfbc767ce308d2edd67a49bd7b764dd07f62f6c
diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
index b6c6f5d306a7..3c9be8d550e0 100755
--- a/scripts/atomic/gen-atomic-fallback.sh
+++ b/scripts/atomic/gen-atomic-fallback.sh
@@ -140,6 +140,32 @@ cat <<EOF
EOF
}

+gen_try_cmpxchg_fallback()
+{
+ local order="$1"; shift;
+
+cat <<EOF
+#ifndef try_cmpxchg${order}
+#define try_cmpxchg${order}(_ptr, _oldp, _new) \\
+({ \\
+ typeof(*ptr) ___r, ___o = *(_oldp); \\
+ ___r = cmpxchg${order}((_ptr), ___o, (_new)); \\
+ if (unlikely(___r != ___o)) \\
+ *(_old) = ___r; \\
+ likely(___r == ___o); \\
+})
+#endif /* try_cmpxchg${order} */
+
+EOF
+}
+
+gen_try_cmpxchg_fallbacks()
+{
+ for order in "" "_acquire" "_release" "_relaxed"; do
+ gen_try_cmpxchg_fallback "${order}"
+ done
+}
+
cat << EOF
// SPDX-License-Identifier: GPL-2.0

@@ -157,6 +183,8 @@ for xchg in "xchg" "cmpxchg" "cmpxchg64"; do
gen_xchg_fallbacks "${xchg}"
done

+gen_try_cmpxchg_fallbacks
+
grep '^[a-z]' "$1" | while read name meta args; do
gen_proto "${meta}" "${name}" "atomic" "int" ${args}
done