[PATCH 2/3] oom: select_bad_process: improve the PF_EXITING check

[Oleg Nesterov <oleg@redhat.com>]

The current PF_EXITING check in select_bad_process() is very limited,
it only works if the task is single-threaded.

Add the new helper which tries to handle the mt case. It is not exactly
clear what should we actually check in this case. This patch checks
PF_EXITING, but perhaps we can take signal_group_exit() into account.
In this case select_bad_process() could detect the exiting process even
before every thread calls exit().

Note:

	- "if (p != current)" check is obviously wrong in mt case too.

	- with or without this change, we should probably check
	  mm->core_state == NULL. We shouldn't assume that is is going
	  to exit "soon" otherwise. But this needs other changes anyway,
	  and in the common case when we do not share ->mm with another
	  process the false positive is not possible.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 mm/oom_kill.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- 38/mm/oom_kill.c~detect_exiting	2011-03-12 14:20:42.000000000 +0100
+++ 38/mm/oom_kill.c	2011-03-12 14:22:19.000000000 +0100
@@ -282,6 +282,22 @@ static enum oom_constraint constrained_a
 }
 #endif
 
+static bool mm_is_exiting(struct task_struct *p)
+{
+	struct task_struct *t;
+	bool has_mm = false;
+
+	t = p;
+	do {
+		if (!(t->flags & PF_EXITING))
+			return false;
+		if (t->mm)
+			has_mm = true;
+	} while_each_thread(p, t);
+
+	return has_mm;
+}
+
 /*
  * Simple selection loop. We chose the process with the highest
  * number of 'points'. We expect the caller will lock the tasklist.
@@ -324,7 +340,7 @@ static struct task_struct *select_bad_pr
 		 * the process of exiting and releasing its resources.
 		 * Otherwise we could get an easy OOM deadlock.
 		 */
-		if (thread_group_empty(p) && (p->flags & PF_EXITING) && p->mm) {
+		if (mm_is_exiting(p)) {
 			if (p != current)
 				return ERR_PTR(-1UL);
 

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