dio_sparse: prevent race between kill and signal

index e93dcd9..376da33 100644
@@ -30,7 +30,10 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <memory.h>
+#include <sys/ipc.h>
 #include <sys/mman.h>
+#include <sys/sem.h>
+#include <sys/types.h>
 #include <sys/wait.h>
 #include <limits.h>
 #include <getopt.h>

@@ -110,6 +113,13 @@ int main(int argc, char **argv)
 	int c;
 	int children_errors = 0;
 	int ret;
+	int sem_id;
+	struct sembuf sem_op;
+	union semun {
+		int val;
+		struct semid_ds *buf;
+		unsigned short int *array;
+	} u;
 
 	while ((c = getopt(argc, argv, "dw:n:a:s:")) != -1) {
 		char *endp;

@@ -145,6 +155,15 @@ int main(int argc, char **argv)
 	}
 
 	setup();
+
+	if ((sem_id = semget(IPC_PRIVATE, 1, IPC_CREAT|S_IRWXU)) == -1)
+		tst_brkm(TBROK|TERRNO, NULL, "allocate semaphore");
+
+	/* semaphore to hold off killing children */
+	u.val = 0;
+	if (semctl(sem_id, 0, SETVAL, u) == -1)
+		tst_brkm(TBROK|TERRNO, NULL, "init semaphore");
+
 	tst_resm(TINFO, "Dirtying free blocks");
 	dirty_freeblocks(filesize);
 

@@ -153,7 +172,15 @@ int main(int argc, char **argv)
 		switch (pid[i] = fork()) {
 		case 0:
 			signal(SIGTERM, SIG_DFL);
+			sem_op = (struct sembuf) {
+				.sem_num = 0,
+				.sem_op = 1,
+				.sem_flg = 0
+			};
+			if (semop(sem_id, &sem_op, 1) == -1)
+				tst_brkm(TBROK|TERRNO, NULL, "raise semaphore");
 			read_sparse(filename, filesize);
+			/* shouldn't be reached, read_sparse calls exit() */
 		break;
 		case -1:
 			while (i-- > 0)

@@ -168,7 +195,16 @@ int main(int argc, char **argv)
 	ret = dio_sparse(filename, alignment, writesize, filesize);
 
 	tst_resm(TINFO, "Killing childrens(s)");
-	
+
+	/* parent waits until all children complete signal() */
+	sem_op = (struct sembuf) {
+		.sem_num = 0,
+		.sem_op = -num_children,
+		.sem_flg = 0
+	};
+	if (semop(sem_id, &sem_op, 1) == -1)
+		tst_brkm(TBROK|TERRNO, NULL, "lower semaphore");
+
 	for (i = 0; i < num_children; i++)
 		kill(pid[i], SIGTERM);
 

@@ -191,7 +227,10 @@ int main(int argc, char **argv)
 		
 	if (!children_errors && !ret)
 		tst_resm(TPASS, "Test passed");
-	
+
+	if (semctl(sem_id, 0, IPC_RMID) == -1)
+		tst_brkm(TBROK|TERRNO, NULL, "remove semaphore");
+
 	cleanup();
 	tst_exit();
 }