Re: [PATCH] LoongArch: Add qspinlock support

[Arnd Bergmann <arnd@arndb.de>]

On Sun, Jun 19, 2022 at 5:48 PM Guo Ren <guoren@kernel.org> wrote:
>
> On Sat, Jun 18, 2022 at 1:40 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > On Sat, Jun 18, 2022 at 1:19 AM Guo Ren <guoren@kernel.org> wrote:
> > >
> > > > static inline u32 arch_xchg32(u32 *ptr, u32 x) {...}
> > > > static inline u64 arch_xchg64(u64 *ptr, u64 x) {...}
> > > >
> > > > #ifdef CONFIG_64BIT
> > > > #define xchg(ptr, x) (sizeof(*ptr) == 8) ? \
> > > >             arch_xchg64((u64*)ptr, (uintptr_t)x)  \
> > > >             arch_xchg32((u32*)ptr, x)
> > > > #else
> > > > #define xchg(ptr, x) arch_xchg32((u32*)ptr, (uintptr_t)x)
> > > > #endif
> > >
> > > The above primitive implies only long & int type args are permitted, right?
> >
> > The idea is to allow any scalar or pointer type, but not structures or
> > unions. If we need to deal with those as well, the macro could be extended
> > accordingly, but I would prefer to limit it as much as possible.
> >
> > There is already cmpxchg64(), which is used for types that are fixed to
> > 64 bit integers even on 32-bit architectures, but it is rarely used except
> > to implement the atomic64_t helpers.
> A lot of 32bit arches couldn't provide cmpxchg64 (like arm's ldrexd/strexd).

Most 32-bit architectures also lack SMP support, so they can fall back to
the generic version from include/asm-generic/cmpxchg-local.h

> Another question: Do you know why arm32 didn't implement
> HAVE_CMPXCHG_DOUBLE with ldrexd/strexd?

I think it's just fairly obscure, the slub code appears to be the only
code that would use it.

> >
> > 80% of the uses of cmpxchg() and xchg() deal with word-sized
> > quantities like 'unsigned long', or 'void *', but the others are almost
> > all fixed 32-bit quantities. We could change those to use cmpxchg32()
> > directly and simplify the cmpxchg() function further to only deal
> > with word-sized arguments, but I would not do that in the first step.
> Don't forget cmpxchg_double for this cleanup, when do you want to
> restart the work?

I have no specific plans at the moment. If you or someone else likes
to look into it, I can dig out my old patch though.

The cmpxchg_double() call seems to already fit in, since it is an
inline function and does not expect arbitrary argument types.

       Arnd