There is a window when de_thread() switches the leader and drops tasklist_lock.
In that window do_each_pid_task(PIDTYPE_PID) finds both new and old leaders.
The problem is pretty much theoretical and probably can be ignored. Currently
the only users of do_each_pid_task(PIDTYPE_PID) are send_sigio/send_sigurg, so
they can send the signal to the same process twice.
Signed-off-by: Oleg Nesterov <[email protected]>
--- PT/include/linux/pid.h~2_do_each_pid_task 2007-10-25 16:22:12.000000000 +0400
+++ PT/include/linux/pid.h 2007-12-07 18:24:21.000000000 +0300
@@ -160,7 +160,13 @@ static inline pid_t pid_vnr(struct pid *
hlist_for_each_entry_rcu((task), pos___, \
&pid->tasks[type], pids[type].node) {
+ /*
+ * Both old and new leaders may be attached to
+ * the same pid in the middle of de_thread().
+ */
#define while_each_pid_task(pid, type, task) \
+ if (type == PIDTYPE_PID) \
+ break; \
} \
} while (0)