On 23/05/17(Tue) 17:41, Mark Kettenis wrote:
> So here is a diff that implements what I proposed recently. This
> recognizes that atomic instructions on amd64 already include an
> implicit memory barrier and allows us to write optimized code that
> avoids a redundant memory barrier.
>
> Note that I don't have a use-case for membar_exit_before_atomic() yet;
> I merely added it for symmetry reasons. I can leave it out if that's
> what people prefer.
>
> This should allow us to use a generic mutex implementation written in
> C without a significant penalty.
This will be used in userland futex-based mutexes as well.
ok mpi@
> Index: sys/atomic.h
> ===================================================================
> RCS file: /cvs/src/sys/sys/atomic.h,v
> retrieving revision 1.4
> diff -u -p -r1.4 atomic.h
> --- sys/atomic.h 24 Jan 2017 22:22:20 -0000 1.4
> +++ sys/atomic.h 23 May 2017 15:01:34 -0000
> @@ -219,4 +219,12 @@ atomic_sub_long_nv(volatile unsigned lon
> #define membar_sync() __sync_synchronize()
> #endif
>
> +#ifndef membar_enter_after_atomic
> +#define membar_enter_after_atomic() membar_enter()
> +#endif
> +
> +#ifndef membar_exit_before_atomic
> +#define membar_exit_before_atomic() membar_exit()
> +#endif
> +
> #endif /* _SYS_ATOMIC_H_ */
> Index: arch/amd64/include/atomic.h
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/include/atomic.h,v
> retrieving revision 1.19
> diff -u -p -r1.19 atomic.h
> --- arch/amd64/include/atomic.h 12 May 2017 08:47:03 -0000 1.19
> +++ arch/amd64/include/atomic.h 23 May 2017 15:02:23 -0000
> @@ -276,6 +276,9 @@ _atomic_sub_long_nv(volatile unsigned lo
> #define membar_sync() __membar("")
> #endif
>
> +#define membar_enter_after_atomic() __membar("")
> +#define membar_exit_before_atomic() __membar("")
> +
> #ifdef _KERNEL
>
> /* virtio needs MP membars even on SP kernels */
>
