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

From: Michael Ellerman
Date: Tue Jul 11 2017 - 08:13:24 EST

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

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)
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)