Re: -current build failure

On Sat, Jul 21, 2012 at 04:00:45PM -0400, Kim Culhan wrote:
> On Fri, Jul 20, 2012 at 11:40 AM, Dimitry Andric <dim_at_freebsd.org> wrote:
> > On 2012-07-20 16:49, Kim Culhan wrote:
> >>  Seeing this for r:238655
> > ...
> >> In file included from /usr/src/sys/modules/dtrace/dtrace/../../../sys/pcpu.h:44:
> >> ./machine/pcpu.h:226:13: error: indirection of non-volatile null
> >> pointer will be deleted, not trap
> >>       [-Werror,-Wnull-dereference]
> >>             : "m" (*(char *)OFFSETOF_CURTHREAD));
> >>                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> >> ./machine/pcpu.h:226:13: note: consider using __builtin_trap() or
> >> qualifying pointer with 'volatile'
> >
> > That's indeed a valid warning from clang, since OFFSETOF_CURTHREAD is
> > usually zero.  It's probably due to recent work on dtrace.  I'm not in
> > the neighborhood of a FreeBSD box right now to verify, but can you
> > please try to change the cast to "(volatile char *)"?  That should fix
> > the warning.
> 
> Yes it did, I know there are many considerations wrt to this warning.

This should be equivalent to what you tried. Can you test build and
boot resulting kernel with this patch ?

diff --git a/sys/amd64/include/pcpu.h b/sys/amd64/include/pcpu.h
index 5d1fd4d..7b3c934 100644
--- a/sys/amd64/include/pcpu.h
+++ b/sys/amd64/include/pcpu.h
_at__at_ -217,16 +217,22 _at__at_ extern struct pcpu *pcpup;
 #define	PCPU_SET(member, val)	__PCPU_SET(pc_ ## member, val)
 
 #define	OFFSETOF_CURTHREAD	0
+#ifdef __clang__
+#define	VOLATILE	volatile
+#else
+#define	VOLATILE
+#endif
 static __inline __pure2 struct thread *
 __curthread(void)
 {
 	struct thread *td;
 
 	__asm("movq %%gs:%1,%0" : "=r" (td)
-	    : "m" (*(char *)OFFSETOF_CURTHREAD));
+	    : "m" (*(VOLATILE char *)OFFSETOF_CURTHREAD));
 	return (td);
 }
 #define	curthread		(__curthread())
+#undef VOLATILE
 
 #define	OFFSETOF_CURPCB		32
 static __inline __pure2 struct pcb *