Re: [PATCH] LoongArch: Add qspinlock support

[Arnd Bergmann <arnd@arndb.de>]

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.

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.

        Arnd