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fix unlock(&procalloc) race on mp systems

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

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

@@ -86,13 +86,15 @@

 			 *	palloc

 			 */

 			mmurelease(up);

+			unlock(&palloc);

 			up->qnext = procalloc.free;

 			procalloc.free = up;

-			unlock(&palloc);

+			/* proc is free now, make sure unlock() wont touch it */

+			up = procalloc.p = nil;

 			unlock(&procalloc);

-			break;

+			sched();

 		}

 		up->mach = nil;

 		updatecpu(up);

-- 

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