Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

Al Viro
2024-05-02T22:08:06Z

On Thu, May 02, 2024 at 02:18:48PM -0700, Paul E. McKenney wrote:

> If you are only ever doing atomic read-modify-write operations on the
> byte in question, then agreed, you don't care about byte loads and stores.
>
> But there are use cases that do mix smp_store_release() with cmpxchg(),
> and those use cases won't work unless at least byte store is implemented.
> Or I suppose that we could use cmpxchg() instead of smp_store_release(),
> but that is wasteful for architectures that do support byte stores.
>
> So EV56 adds the byte loads and stores needed for those use cases.
>
> Or am I missing your point?

arch/alpha/include/cmpxchg.h:
#define arch_cmpxchg(ptr, o, n) \
({ \
__typeof__(*(ptr)) __ret; \
__typeof__(*(ptr)) _o_ = (o); \
__typeof__(*(ptr)) _n_ = (n); \
smp_mb(); \
__ret = (__typeof__(*(ptr))) __cmpxchg((ptr), \
(unsigned long)_o_, (unsigned long)_n_, sizeof(*(ptr)));\
smp_mb(); \
__ret; \
})

Are those smp_mb() in there enough?

I'm probably missing your point, though - what mix of cmpxchg and
smp_store_release on 8bit values?