Re: [PATCH] locking/atomic: Make test_and_*_bit() ordered on failure

[Arnd Bergmann]

On Tue, Aug 16, 2022 at 9:03 AM Hector Martin <marcan@xxxxxxxxx> wrote:
>
> These operations are documented as always ordered in
> include/asm-generic/bitops/instrumented-atomic.h, and producer-consumer
> type use cases where one side needs to ensure a flag is left pending
> after some shared data was updated rely on this ordering, even in the
> failure case.
>
> This is the case with the workqueue code, which currently suffers from a
> reproducible ordering violation on Apple M1 platforms (which are
> notoriously out-of-order) that ends up causing the TTY layer to fail to
> deliver data to userspace properly under the right conditions. This
> change fixes that bug.
>
> Change the documentation to restrict the "no order on failure" story to
> the _lock() variant (for which it makes sense), and remove the
> early-exit from the generic implementation, which is what causes the
> missing barrier semantics in that case. Without this, the remaining
> atomic op is fully ordered (including on ARM64 LSE, as of recent
> versions of the architecture spec).
>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: e986a0d6cb36 ("locking/atomics, asm-generic/bitops/atomic.h: Rewrite using atomic_*() APIs")
> Fixes: 61e02392d3c7 ("locking/atomic/bitops: Document and clarify ordering semantics for failed test_and_{}_bit()")
> Signed-off-by: Hector Martin <marcan@xxxxxxxxx>
> ---
>  Documentation/atomic_bitops.txt     | 2 +-
>  include/asm-generic/bitops/atomic.h | 6 ------

I double-checked all the architecture specific implementations to ensure
that the asm-generic one is the only one that needs the fix.

I assume this gets merged through the locking tree or that Linus picks it up
directly, not through my asm-generic tree.

Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>