[RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using atomic_fetch_*

On Thu, Feb 15, 2018 at 06:20:49PM +0000, Will Deacon wrote:

quoted

The only other comment is that I think it would be better if you use
atomic_t instead of atomic_long_t. It would just mean changing
BIT_WORD() and BIT_MASK().

It would make it pretty messy for big-endian architectures, I think...

Urgh, the big.little indians strike again.. Bah I always forget about
that.

#define BIT_U32_MASK(nr)	(1UL << ((nr) % 32))
#define BIT_U32_WORD(nr)	(((nr) / 32) ^ (4 * __BIG_ENDIAN__))

Or something like that might work, but I always get these things wrong.

quoted

The reason is that we generate a pretty sane set of atomic_t primitives
as long as the architecture supplies cmpxchg, but atomic64 defaults to
utter crap, even on 64bit platforms.

I think all the architectures using this today are 32-bit:

blackfin
c6x
cris
metag
openrisc
sh
xtensa

and I don't know how much we should care about optimising the generic atomic
bitops for 64-bit architectures that rely on spinlocks for 64-bit atomics!

You're probably right, but it just bugs me that we default to such
horrible crap. Arguably we should do a better default for atomic64_t on
64bit archs. But that's for another time.