| Message ID | 20241119112009.1286675-1-ptesarik@suse.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [1/1] mm/rodata_test: Use READ_ONCE() to read const variable | expand |
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; > }
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; }
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(-)