[PATCH] mm: fix mm_struct reference counting bugs in mm/oom_kill.c

Fix oom_kill_task() so it doesn't call mmput() (which may sleep) while
holding tasklist_lock.

Signed-off-by: David S. Peterson <dsp@llnl.gov>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 23 insertions, 22 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 9a643c4bf77..042e6436c3e 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -254,17 +254,24 @@ static void __oom_kill_task(task_t *p, const char *message)
         force_sig(SIGKILL, p);
 }
 
-static struct mm_struct *oom_kill_task(task_t *p, const char *message)
+static int oom_kill_task(task_t *p, const char *message)
 {
-        struct mm_struct *mm = get_task_mm(p);
+        struct mm_struct *mm;
         task_t * g, * q;
 
-        if (!mm)
-                return NULL;
-        if (mm == &init_mm) {
-                mmput(mm);
-                return NULL;
-        }
+        mm = p->mm;
+
+        /* WARNING: mm may not be dereferenced since we did not obtain its
+         * value from get_task_mm(p).  This is OK since all we need to do is
+         * compare mm to q->mm below.
+         *
+         * Furthermore, even if mm contains a non-NULL value, p->mm may
+         * change to NULL at any time since we do not hold task_lock(p).
+         * However, this is of no concern to us.
+         */
+
+        if (mm == NULL || mm == &init_mm)
+                return 1;
 
         __oom_kill_task(p, message);
         /*
@@ -276,13 +283,12 @@ static struct mm_struct *oom_kill_task(task_t *p, const char *message)
                         __oom_kill_task(q, message);
         while_each_thread(g, q);
 
-        return mm;
+        return 0;
 }
 
-static struct mm_struct *oom_kill_process(struct task_struct *p,
-                                unsigned long points, const char *message)
+static int oom_kill_process(struct task_struct *p, unsigned long points,
+                const char *message)
 {
-        struct mm_struct *mm;
         struct task_struct *c;
         struct list_head *tsk;
 
@@ -293,9 +299,8 @@ static struct mm_struct *oom_kill_process(struct task_struct *p,
                 c = list_entry(tsk, struct task_struct, sibling);
                 if (c->mm == p->mm)
                         continue;
-                mm = oom_kill_task(c, message);
-                if (mm)
-                        return mm;
+                if (!oom_kill_task(c, message))
+                        return 0;
         }
         return oom_kill_task(p, message);
 }
@@ -310,7 +315,6 @@ static struct mm_struct *oom_kill_process(struct task_struct *p,
  */
 void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
 {
-        struct mm_struct *mm = NULL;
         task_t *p;
         unsigned long points = 0;
 
@@ -330,12 +334,12 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
          */
         switch (constrained_alloc(zonelist, gfp_mask)) {
         case CONSTRAINT_MEMORY_POLICY:
-                mm = oom_kill_process(current, points,
+                oom_kill_process(current, points,
                                 "No available memory (MPOL_BIND)");
                 break;
 
         case CONSTRAINT_CPUSET:
-                mm = oom_kill_process(current, points,
+                oom_kill_process(current, points,
                                 "No available memory in cpuset");
                 break;
 
@@ -357,8 +361,7 @@ retry:
                         panic("Out of memory and no killable processes...\n");
                 }
 
-                mm = oom_kill_process(p, points, "Out of memory");
-                if (!mm)
+                if (oom_kill_process(p, points, "Out of memory"))
                         goto retry;
 
                 break;
@@ -367,8 +370,6 @@ retry:
 out:
         read_unlock(&tasklist_lock);
         cpuset_unlock();
-        if (mm)
-                mmput(mm);
 
         /*
          * Give "p" a good chance of killing itself before we