my solution to lab5

lab/user/testpiperace.c

@@ -0,0 +1,66 @@
+#include <inc/lib.h>
+
+void
+umain(int argc, char **argv)
+{
+	int p[2], r, pid, i, max;
+	void *va;
+	struct Fd *fd;
+	const volatile struct Env *kid;
+
+	cprintf("testing for dup race...\n");
+	if ((r = pipe(p)) < 0)
+		panic("pipe: %e", r);
+	max = 200;
+	if ((r = fork()) < 0)
+		panic("fork: %e", r);
+	if (r == 0) {
+		close(p[1]);
+		//
+		// Now the ref count for p[0] will toggle between 2 and 3
+		// as the parent dups and closes it (there's a close implicit in dup).
+		//
+		// The ref count for p[1] is 1.
+		// Thus the ref count for the underlying pipe structure
+		// will toggle between 3 and 4.
+		//
+		// If a clock interrupt catches close between unmapping
+		// the pipe structure and unmapping the fd, we'll have
+		// a ref count for p[0] of 3, a ref count for p[1] of 1,
+		// and a ref count for the pipe structure of 3, which is
+		// a no-no.
+		//
+		// If a clock interrupt catches dup between mapping the
+		// fd and mapping the pipe structure, we'll have the same
+		// ref counts, still a no-no.
+		//
+		for (i=0; i<max; i++) {
+			if(pipeisclosed(p[0])){
+				cprintf("RACE: pipe appears closed\n");
+				exit();
+			}
+			sys_yield();
+		}
+		// do something to be not runnable besides exiting
+		ipc_recv(0,0,0);
+	}
+	pid = r;
+	cprintf("pid is %d\n", pid);
+	va = 0;
+	kid = &envs[ENVX(pid)];
+	cprintf("kid is %d\n", kid-envs);
+	dup(p[0], 10);
+	while (kid->env_status == ENV_RUNNABLE)
+		dup(p[0], 10);
+
+	cprintf("child done with loop\n");
+	if (pipeisclosed(p[0]))
+		panic("somehow the other end of p[0] got closed!");
+	if ((r = fd_lookup(p[0], &fd)) < 0)
+		panic("cannot look up p[0]: %e", r);
+	va = fd2data(fd);
+	if (pageref(va) != 3+1)
+		cprintf("\nchild detected race\n");
+	else
+		cprintf("\nrace didn't happen\n", max);
+}