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

[Marc Zyngier <maz@kernel.org>]

On Tue, 16 Aug 2022 14:05:54 +0100,
Jon Nettleton <jon@solid-run.com> wrote:
> 
> On Tue, Aug 16, 2022 at 3:01 PM Will Deacon <will@kernel.org> wrote:
> >
> > On Tue, Aug 16, 2022 at 02:29:49PM +0200, Jon Nettleton wrote:
> > > On Tue, Aug 16, 2022 at 10:17 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > > >
> > > > On Tue, Aug 16, 2022 at 9:03 AM Hector Martin <marcan@marcan.st> 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@linux-foundation.org>
> > > > > Cc: stable@vger.kernel.org
> > > > > 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@marcan.st>
> > > > > ---
> > > > >  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@arndb.de>
> > > >
> > > > _______________________________________________
> > > > linux-arm-kernel mailing list
> > > > linux-arm-kernel@lists.infradead.org
> > > > 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
> >
> > Hmm, that should only exist if LOCKDEP is enabled and performance tends to
> > go out of the window if you have that on. Can you reproduce the same
> > regression with lockdep disabled?
> >
> > Will
> 
> Yep I am working on it.  We should note that
> 
> config LOCKDEP_SUPPORT
>         def_bool y
> 
> is the default for arm64

Yes, as the architecture supports LOCKDEP. However, you probably have
something like CONFIG_PROVE_LOCKING to see such a performance hit (and
that's definitely not on by default).

	M.

-- 
Without deviation from the norm, progress is not possible.