The C compiler may optimize away the memory read of a const variable if its
value is known at compile time.
In particular, GCC14 with -O2 generates no code at all for test 1, and it
generates the following x86_64 instructions for test 3:
cmpl $195, 4(%rsp)
je .L14
That is, it replaces the read of rodata_test_data with an immediate value
and compares it to the value of the local variable "zero".
Use READ_ONCE() to undo any such compiler optimizations and enforce a
memory read.
Fixes: 2959a5f726f6 ("mm: add arch-independent testcases for RODATA")
Signed-off-by: Petr Tesarik <ptesarik@xxxxxxxx>
---
mm/rodata_test.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/rodata_test.c b/mm/rodata_test.c
index 6d783436951f..3b60425d80fe 100644
--- a/mm/rodata_test.c
+++ b/mm/rodata_test.c
@@ -20,7 +20,7 @@ void rodata_test(void)
/* test 1: read the value */
/* If this test fails, some previous testrun has clobbered the state */
- if (!rodata_test_data) {
+ if (!READ_ONCE(rodata_test_data)) {
pr_err("test 1 fails (start data)\n");
return;
}
@@ -33,7 +33,7 @@ void rodata_test(void)
}
/* test 3: check the value hasn't changed */
- if (rodata_test_data == zero) {
+ if (READ_ONCE(rodata_test_data) == zero) {
pr_err("test data was changed\n");
return;
}
--
2.46.1
