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

[Jon Nettleton]

On Tue, Aug 16, 2022 at 10:17 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> 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>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Testing this patch on pre Armv8.1 specifically Cortex-A72 and
Cortex-A53 cores I am seeing
a huge performance drop with this patch applied. Perf is showing
lock_is_held_type() as the worst offender
but that could just be the function getting blamed. The most obvious
indicator of the performance loss is
ssh throughput.  With the patch I am only able to achieve around
20MB/s and without the patch I am able
to transfer around 112MB/s, no other changes.

When I have more time I can do some more in depth testing, but for now
I just wanted to bring this
issue up so perhaps others can chime in regarding how it performs on
their hardware.

-Jon