Re: [PATCH 1/1] mm/rodata_test: Use READ_ONCE() to read const variable

[Petr Tesarik <ptesarik@suse.com>]

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@suse.com> 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@suse.com>
> ---
>  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;
>  	}