On Tue, 2024-07-09 at 16:16 +0200, Arnd Bergmann wrote:
On Tue, Jul 9, 2024, at 15:45, Geert Uytterhoeven wrote:
On Tue, Jul 9, 2024 at 1:58 PM Jeff Layton <jlayton@xxxxxxxxxx>
wrote:
I've been getting some of these warning emails from the KTR. I
think
this is in reference to this patch, which adds a 64-bit
try_cmpxchg in
the timestamp handling code:
https://lore.kernel.org/linux-fsdevel/20240708-mgtime-v4-0-a0f3c6fb57f3@xxxxxxxxxx/
On m68k, there is a prototype for __invalid_cmpxchg_size, but no
actual
function, AFAICT. Should that be defined somewhere, or is this a
deliberate way to force a build break in this case?
It's a deliberate way to break the build.
More to the point though: do I need to do anything special for
m86k
here (or for other arches that can't do a native 64-bit cmpxchg)?
64-bit cmpxchg() is only guaranteed to exist on 64-bit platforms.
See also
https://elixir.bootlin.com/linux/latest/source/include/asm-generic/cmpxchg.h#L62
I think you can use arch_cmpxchg64(), though.
arch_cmpxchg64() is an internal helper provided by some
architectures. Driver code should use cmpxchg64() for
the explicitly 64-bit sized atomic operation.
I'm fairly sure we still don't provide this across all
32-bit architectures though: on architectures that have
64-bit atomics (i686, armv6k, ...) these can be provided
architecture specific code, and on non-SMP kernels they
can use the generic fallback through
generic_cmpxchg64_local(), but on SMP architectures without
native atomics you need a Kconfig dependency to turn off
the particular code.
I think the simplest solution is to make the floor value I'm tracking
be an atomic64_t. That looks like it should smooth over the differences
between arches. I'm testing a patch to do that now.
Thanks!
--
Jeff Layton <jlayton@xxxxxxxxxx>
