use acquire/release to force order for pid=np->pid;np->state=RUNNING

for bug reported by symingz@gmail.com and cs1100254@cse.iitd.ernet.in

proc.c

@@ -153,10 +153,16 @@ fork(void)
     if(proc->ofile[i])
       np->ofile[i] = filedup(proc->ofile[i]);
   np->cwd = idup(proc->cwd);
+
+  safestrcpy(np->name, proc->name, sizeof(proc->name));
  
   pid = np->pid;
+
+  // lock to force the compiler to emit the np->state write last.
+  acquire(&ptable.lock);
   np->state = RUNNABLE;
-  safestrcpy(np->name, proc->name, sizeof(proc->name));
+  release(&ptable.lock);
+  
   return pid;
 }
 
@@ -455,5 +461,3 @@ procdump(void)
     cprintf("\n");
   }
 }
-
-