[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>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 9a643c4..042e643 100644 (file)
--- 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