kernel/pid.c: improve flow of a loop inside alloc_pidmap.

find_next_offset() searches for an available "cleaned bit" in the
respective pid bitmap (page), so returns the offset if found, otherwise
it returns a value equals to BITS_PER_PAGE.

For example, suppose find_next_offset didn't find any available bit, so
there's no purpose to call mk_pid (Wasteful Cpu Cycles).

Therefore, I found it could be better to call mk_pid after the checking
(offset < BITS_PER_PAGE) returned sucessfully! Another point: If (offset
< BITS_PER_PAGE) results in a "failure", then mk_pid would be called
again afterwards.

[akpm@linux-foundation.org: simplify code]
Signed-off-by: NRaphael S. Carvalho <raphael.scarv@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/pid.c

@@ -183,15 +183,19 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 				break;
 		}
 		if (likely(atomic_read(&map->nr_free))) {
-			do {
+			for ( ; ; ) {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
 					set_last_pid(pid_ns, last, pid);
 					return pid;
 				}
 				offset = find_next_offset(map, offset);
+				if (offset >= BITS_PER_PAGE)
+					break;
 				pid = mk_pid(pid_ns, map, offset);
-			} while (offset < BITS_PER_PAGE && pid < pid_max);
+				if (pid >= pid_max)
+					break;
+			}
 		}
 		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 			++map;