[PATCH] oom-kill: mm locking fix

Dave Peterson <dsp@llnl.gov> points out that badness() is playing with
mm_structs without taking a reference on them.

mmput() can sleep, so taking a reference here (inside tasklist_lock) is
hard.  Fix it up via task_lock() instead.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 18 insertions, 8 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 78747afad6b..9a643c4bf77 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -46,15 +46,25 @@
 unsigned long badness(struct task_struct *p, unsigned long uptime)
 {
         unsigned long points, cpu_time, run_time, s;
-        struct list_head *tsk;
+        struct mm_struct *mm;
+        struct task_struct *child;
 
-        if (!p->mm)
+        task_lock(p);
+        mm = p->mm;
+        if (!mm) {
+                task_unlock(p);
                 return 0;
+        }
 
         /*
          * The memory size of the process is the basis for the badness.
          */
-        points = p->mm->total_vm;
+        points = mm->total_vm;
+
+        /*
+         * After this unlock we can no longer dereference local variable `mm'
+         */
+        task_unlock(p);
 
         /*
          * Processes which fork a lot of child processes are likely
@@ -64,11 +74,11 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
          * child is eating the vast majority of memory, adding only half
          * to the parents will make the child our kill candidate of choice.
          */
-        list_for_each(tsk, &p->children) {
-                struct task_struct *chld;
-                chld = list_entry(tsk, struct task_struct, sibling);
-                if (chld->mm != p->mm && chld->mm)
-                        points += chld->mm->total_vm/2 + 1;
+        list_for_each_entry(child, &p->children, sibling) {
+                task_lock(child);
+                if (child->mm != mm && child->mm)
+                        points += child->mm->total_vm/2 + 1;
+                task_unlock(child);
         }
 
         /*