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SubjectRe: [PATCH] wait_on_bit: add an acquire memory barrier
On Thu, Aug 25, 2022 at 05:03:40PM -0400, Mikulas Patocka wrote:
> Here I reworked your patch, so that test_bit_acquire is defined just like
> test_bit. There's some code duplication (in
> include/asm-generic/bitops/generic-non-atomic.h and in
> arch/x86/include/asm/bitops.h), but that duplication exists in the
> test_bit function too.
>
> I tested it on x86-64 and arm64. On x86-64 it generates the "bt"
> instruction for variable-bit test and "shr; and $1" for constant bit test.
> On arm64 it generates the "ldar" instruction for both constant and
> variable bit test.
>
> For me, the kernel 6.0-rc2 doesn't boot in an arm64 virtual machine at all
> (with or without this patch), so I only compile-tested it on arm64. I have
> to bisect it.

It's working fine for me and I haven't had any other reports that it's not
booting. Please could you share some more details about your setup so we
can try to reproduce the problem?

> From: Mikulas Patocka <mpatocka@redhat.com>
>
> There are several places in the kernel where wait_on_bit is not followed
> by a memory barrier (for example, in drivers/md/dm-bufio.c:new_read). On
> architectures with weak memory ordering, it may happen that memory
> accesses that follow wait_on_bit are reordered before wait_on_bit and they
> may return invalid data.
>
> Fix this class of bugs by introducing a new function "test_bit_acquire"
> that works like test_bit, but has acquire memory ordering semantics.
>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Cc: stable@vger.kernel.org
>
> arch/x86/include/asm/bitops.h | 13 +++++++++++++
> include/asm-generic/bitops/generic-non-atomic.h | 14 ++++++++++++++
> include/asm-generic/bitops/instrumented-non-atomic.h | 12 ++++++++++++
> include/asm-generic/bitops/non-atomic.h | 1 +
> include/asm-generic/bitops/non-instrumented-non-atomic.h | 1 +
> include/linux/bitops.h | 1 +
> include/linux/buffer_head.h | 2 +-
> include/linux/wait_bit.h | 8 ++++----
> kernel/sched/wait_bit.c | 2 +-
> 9 files changed, 48 insertions(+), 6 deletions(-)

This looks good to me, thanks for doing it! Just one thing that jumped out
at me:

> Index: linux-2.6/include/linux/buffer_head.h
> ===================================================================
> --- linux-2.6.orig/include/linux/buffer_head.h
> +++ linux-2.6/include/linux/buffer_head.h
> @@ -156,7 +156,7 @@ static __always_inline int buffer_uptoda
> * make it consistent with folio_test_uptodate
> * pairs with smp_mb__before_atomic in set_buffer_uptodate
> */
> - return (smp_load_acquire(&bh->b_state) & (1UL << BH_Uptodate)) != 0;
> + return test_bit_acquire(BH_Uptodate, &bh->b_state);

Do you think it would be worth adding set_bit_release() and then relaxing
set_buffer_uptodate() to use that rather than the smp_mb__before_atomic()?

Will

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