lib/atomic64_test.c: add a test that atomic64_inc_not_zero() returns an int

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.

Link: http://lkml.kernel.org/r/1499775133-1231-1-git-send-email-mpe@ellerman.id.au
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Douglas Miller <dougmill@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/atomic64_test.c b/lib/atomic64_test.c
index fd70c0e0e67314999fcab5c69c9c06dcec527c03..62ab629f51ca2d6a05887f41167aaeeee3cd202a 100644 (file)
--- 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)