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kernel: insert memory barrier in the scheduler before setting up->mach = nil

we have to ensure that all stores saving the process state
have completed before setting up->mach = nil in the scheduler.
otherwise, another cpu could observe up->mach == nil while
the stores such as the processes p->sched label have not finnished.

--- a/sys/src/9/port/proc.c

+++ b/sys/src/9/port/proc.c

@@ -70,6 +70,9 @@

 			edfrecord(up);

 		m->proc = nil;

 		switch(up->state) {

+		default:

+			updatecpu(up);

+			break;

 		case Running:

 			ready(up);

 			break;

@@ -88,8 +91,8 @@

 			mmurelease(up);

 			unlock(&palloc);

-			up->mach = nil;

 			updatecpu(up);

+			up->mach = nil;

 			up->qnext = procalloc.free;

 			procalloc.free = up;

@@ -99,8 +102,8 @@

 			unlock(&procalloc);

 			sched();

 		}

+		coherence();

 		up->mach = nil;

-		updatecpu(up);

 		up = nil;

 	}

 	sched();

-- 

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