csky: atomic: Add custom atomic.h implementation
The generic atomic.h used cmpxchg to implement the atomic operations, it will cause daul loop to reduce the forward guarantee. The patch implement csky custom atomic operations with ldex/stex instructions for the best performance. Important comment by Rutland: 8e86f0b4 ("arm64: atomics: fix use of acquire + release for full barrier semantics") Link: https://lore.kernel.org/linux-riscv/CAJF2gTSAxpAi=LbAdu7jntZRUa=-dJwL0VfmDfBV5MHB=rcZ-w@mail.gmail.com/T/#m27a0f1342995deae49ce1d0e1f2683f8a181d6c3 Signed-off-by: Guo Ren <guoren@linux.alibaba.com> Signed-off-by: Guo Ren <guoren@kernel.org> Cc: Mark Rutland <mark.rutland@arm.com>
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