[PATCH v2 1/4] tools/include: Implement a couple of atomic_t ops

[Brendan Jackman]

The VMA tests need an operation equivalent to
atomic_inc_unless_negative() to implement a fake mapping_map_writable().
Adding it will enable them to switch to the shared atomic headers and
simplify that fake implementation.

In order to add that, also add atomic_try_cmpxchg() which can be used to
implement it. This is copied from Documentation/atomic_t.txt. Then,
implement atomic_inc_unless_negative() itself based on the
raw_atomic_dec_unless_positive() in
include/linux/atomic/atomic-arch-fallback.h.

There's no present need for a highly-optimised version of this (nor any
reason to think this implementation is sub-optimal on x86) so just
implement this with generic C, no x86-specifics.

Signed-off-by: Brendan Jackman <jackmanb at google.com>
---
 tools/include/linux/atomic.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/include/linux/atomic.h b/tools/include/linux/atomic.h
index 01907b33537e04c5e860dd2cd61a61dfab6ea8f1..50c66ba9ada58cf05125e2e2472734bf3b0f8595 100644
--- a/tools/include/linux/atomic.h
+++ b/tools/include/linux/atomic.h
@@ -12,4 +12,26 @@ void atomic_long_set(atomic_long_t *v, long i);
 #define  atomic_cmpxchg_release         atomic_cmpxchg
 #endif /* atomic_cmpxchg_relaxed */
 
+static inline 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;
+}
+
+static inline bool atomic_inc_unless_negative(atomic_t *v)
+{
+	int c = atomic_read(v);
+
+	do {
+		if (unlikely(c < 0))
+			return false;
+	} while (!atomic_try_cmpxchg(v, &c, c + 1));
+
+	return true;
+}
+
 #endif /* __TOOLS_LINUX_ATOMIC_H */

-- 
2.50.1