[PATCH] oom: fix the unsafe proc_oom_score()->badness() call

[Oleg Nesterov]

proc_oom_score(task) have a reference to task_struct, but that is all.
If this task was already released before we take tasklist_lock

	- we can't use task->group_leader, it points to nowhere

	- it is not safe to call badness() even if this task is
	  ->group_leader, has_intersects_mems_allowed() assumes
	  it is safe to iterate over ->thread_group list.

Add the pid_alive() check to ensure __unhash_process() was not called.

Note: I think we shouldn't use ->group_leader, badness() should return
the same result for any sub-thread. However this is not true currently,
and I think that ->mm check and list_for_each_entry(p->children) in
badness are not right.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---

 fs/proc/base.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- 34-rc1/fs/proc/base.c~OOM_SCORE	2010-03-22 16:36:28.000000000 +0100
+++ 34-rc1/fs/proc/base.c	2010-03-30 18:23:50.000000000 +0200
@@ -430,12 +430,13 @@ static const struct file_operations proc
 unsigned long badness(struct task_struct *p, unsigned long uptime);
 static int proc_oom_score(struct task_struct *task, char *buffer)
 {
-	unsigned long points;
+	unsigned long points = 0;
 	struct timespec uptime;
 
 	do_posix_clock_monotonic_gettime(&uptime);
 	read_lock(&tasklist_lock);
-	points = badness(task->group_leader, uptime.tv_sec);
+	if (pid_alive(task))
+		points = badness(task->group_leader, uptime.tv_sec);
 	read_unlock(&tasklist_lock);
 	return sprintf(buffer, "%lu\n", points);
 }

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