[PATCH] Fix a race in pid generation that causes pids to be reused immediately.

[Salman <sqazi@google.com>]

A program that repeatedly forks and waits is susceptible to having the
same pid repeated, especially when it competes with another instance of the
same program.  This is really bad for bash implementation.  Furthermore, many shell
scripts assume that pid numbers will not be used for some length of time.

Thanks to Ted Tso for the key ideas of this implementation.

Signed-off-by: Salman Qazi <sqazi@google.com>
---
 kernel/pid.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index e9fd8c1..8cedeab 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -153,8 +153,17 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 		if (likely(atomic_read(&map->nr_free))) {
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
+					int prev;
 					atomic_dec(&map->nr_free);
-					pid_ns->last_pid = pid;
+
+					do {
+						prev = last;
+						last = cmpxchg(&pid_ns->last_pid,
+							       prev, pid);
+						if (last >= pid)
+							break;
+					} while (prev != last);
+
 					return pid;
 				}
 				offset = find_next_offset(map, offset);