Oops, this should have been a two-patch series. Will resend in a
minute. Sorry.
Petr T
On Tue, 19 Nov 2024 12:20:09 +0100
Petr Tesarik <ptesarik@xxxxxxxx> wrote:
> 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;
> }
