> Date: Tue, 23 May 2017 17:58:56 +0200
> From: Mike Belopuhov <m...@belopuhov.com>
>
> On Tue, May 23, 2017 at 17:41 +0200, 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.
> >
> > ok?
> >
>
> LGTM, but shouldn't the same thing be done to i386?
Yes.
> > 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 */
> >
>
