Re: [PATCH V8 07/22] LoongArch: Add atomic/locking headers

[Huacai Chen]

Hi, Arnd,

On Mon, Mar 21, 2022 at 5:42 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Sat, Mar 19, 2022 at 3:31 PM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote:
> > LoongArch has no native sub-word xchg/cmpxchg instructions now, but
> > LoongArch-based CPUs support NUMA (e.g., quad-core Loongson-3A5000
> > supports as many as 16 nodes, 64 cores in total). So, we emulate sub-
> > word xchg/cmpxchg in software and use qspinlock/qrwlock rather than
> > ticket locks.
> ...
> > +extern unsigned long __xchg_small(volatile void *ptr, unsigned long x,
> > +                                 unsigned int size);
> > +
> > +static inline unsigned long __xchg(volatile void *ptr, unsigned long x,
> > +                                  int size)
> > +{
> > +       switch (size) {
> > +       case 1:
> > +       case 2:
> > +               return __xchg_small(ptr, x, size);
> > +
>
> I think it's better to not define the "small" versions at all, since they are
> inefficient and probably not safe to use for the few things that try to call
> them, such as the qspinlock implementation.
>
> I have an experimental patch set that removes these from the kernel
> altogether and makes xchg()/cmpxchg() only work on 32-bit or
> 64-bit values.
>
> > diff --git a/arch/loongarch/include/asm/spinlock.h b/arch/loongarch/include/asm/spinlock.h
> > new file mode 100644
> > index 000000000000..7cb3476999be
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/spinlock.h
> > +
> > +#include <asm/processor.h>
> > +#include <asm/qspinlock.h>
> > +#include <asm/qrwlock.h>
> > +
>
> There is a patch series from Peter Zijlstra, Palmer Dabbelt and Guo Ren
> that is currently under review for risc-v and csky, to add a generic ticket lock
> implementation that does not rely on sub-word atomics [1]. I think we
> also want to convert mips, xtensa, openrisc, and sparc64 to use that,
> since they have the same issue with the lack of 16-bit atomics.
>
> Please coordinate the inclusion of the patches with them and use that
> spinlock implementation for the initial merge, to avoid further discussion
> on the topic. If at a later point you are able to come up with a qspinlock
> implementation that has convincing forward-progress guarantees and
> can be shown to be better, we can revisit this.
In my opinion, forward-progress is solved in V2, since we have
reworked __xchg_small()/__cmpxchg_small(), and qspinlock is needed by
NUMA.
However, if the generic ticket lock is merged later, I will try to use
it at present.

Huacai
>
>       Arnd
>
> [1] https://lore.kernel.org/lkml/20220319035457.2214979-1-guoren@xxxxxxxxxx/