[PATCH] lib/atomic64-test: Add a test that atomic64_inc_not_zero() returns an int

[Michael Ellerman]

atomic64_inc_not_zero() returns a "truth value" which in C is traditionally an
int. That means callers are likely to expect the result will fit in an int.

If an implementation returns a "true" value which does not fit in an int, then
there's a possibility that callers will truncate it when they store it in an
int. In fact this happened in practice, see commit 966d2b04e070
("percpu-refcount: fix reference leak during percpu-atomic transition").

So add a test that the result fits in an int, even when the input doesn't. This
catches the case where an implementation just passes the non-zero input value
out as the result.

Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
---
 lib/atomic64_test.c | 7 +++++++
 1 file changed, 7 insertions(+)

Sending this to you Andrew because it doesn't seem to have any other
maintainer.

diff --git a/lib/atomic64_test.c b/lib/atomic64_test.c
index fd70c0e0e673..62ab629f51ca 100644
--- a/lib/atomic64_test.c
+++ b/lib/atomic64_test.c
@@ -153,8 +153,10 @@ static __init void test_atomic64(void)
 	long long v0 = 0xaaa31337c001d00dLL;
 	long long v1 = 0xdeadbeefdeafcafeLL;
 	long long v2 = 0xfaceabadf00df001LL;
+	long long v3 = 0x8000000000000000LL;
 	long long onestwos = 0x1111111122222222LL;
 	long long one = 1LL;
+	int r_int;
 
 	atomic64_t v = ATOMIC64_INIT(v0);
 	long long r = v0;
@@ -240,6 +242,11 @@ static __init void test_atomic64(void)
 	BUG_ON(!atomic64_inc_not_zero(&v));
 	r += one;
 	BUG_ON(v.counter != r);
+
+	/* Confirm the return value fits in an int, even if the value doesn't */
+	INIT(v3);
+	r_int = atomic64_inc_not_zero(&v);
+	BUG_ON(!r_int);
 }
 
 static __init int test_atomics_init(void)
-- 
2.7.4