memcg: avoid deadlock between move charge and try_charge()
Daisuke Nishimura [Wed, 24 Nov 2010 20:57:06 +0000 (12:57 -0800)]
__mem_cgroup_try_charge() can be called under down_write(&mmap_sem)(e.g.
mlock does it). This means it can cause deadlock if it races with move charge:

Ex.1)
                move charge             |        try charge
  --------------------------------------+------------------------------
    mem_cgroup_can_attach()             |  down_write(&mmap_sem)
      mc.moving_task = current          |    ..
      mem_cgroup_precharge_mc()         |  __mem_cgroup_try_charge()
        mem_cgroup_count_precharge()    |    prepare_to_wait()
          down_read(&mmap_sem)          |    if (mc.moving_task)
          -> cannot aquire the lock     |    -> true
                                        |      schedule()

Ex.2)
                move charge             |        try charge
  --------------------------------------+------------------------------
    mem_cgroup_can_attach()             |
      mc.moving_task = current          |
      mem_cgroup_precharge_mc()         |
        mem_cgroup_count_precharge()    |
          down_read(&mmap_sem)          |
          ..                            |
          up_read(&mmap_sem)            |
                                        |  down_write(&mmap_sem)
    mem_cgroup_move_task()              |    ..
      mem_cgroup_move_charge()          |  __mem_cgroup_try_charge()
        down_read(&mmap_sem)            |    prepare_to_wait()
        -> cannot aquire the lock       |    if (mc.moving_task)
                                        |    -> true
                                        |      schedule()

To avoid this deadlock, we do all the move charge works (both can_attach() and
attach()) under one mmap_sem section.
And after this patch, we set/clear mc.moving_task outside mc.lock, because we
use the lock only to check mc.from/to.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

index 62d1880..26218df 100644 (file)
@@ -278,13 +278,14 @@ enum move_type {
 
 /* "mc" and its members are protected by cgroup_mutex */
 static struct move_charge_struct {
-       spinlock_t        lock; /* for from, to, moving_task */
+       spinlock_t        lock; /* for from, to */
        struct mem_cgroup *from;
        struct mem_cgroup *to;
        unsigned long precharge;
        unsigned long moved_charge;
        unsigned long moved_swap;
        struct task_struct *moving_task;        /* a task moving charges */
+       struct mm_struct *mm;
        wait_queue_head_t waitq;                /* a waitq for other context */
 } mc = {
        .lock = __SPIN_LOCK_UNLOCKED(mc.lock),
@@ -4631,7 +4632,7 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm)
        unsigned long precharge;
        struct vm_area_struct *vma;
 
-       down_read(&mm->mmap_sem);
+       /* We've already held the mmap_sem */
        for (vma = mm->mmap; vma; vma = vma->vm_next) {
                struct mm_walk mem_cgroup_count_precharge_walk = {
                        .pmd_entry = mem_cgroup_count_precharge_pte_range,
@@ -4643,7 +4644,6 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm)
                walk_page_range(vma->vm_start, vma->vm_end,
                                        &mem_cgroup_count_precharge_walk);
        }
-       up_read(&mm->mmap_sem);
 
        precharge = mc.precharge;
        mc.precharge = 0;
@@ -4694,11 +4694,16 @@ static void mem_cgroup_clear_mc(void)
 
                mc.moved_swap = 0;
        }
+       if (mc.mm) {
+               up_read(&mc.mm->mmap_sem);
+               mmput(mc.mm);
+       }
        spin_lock(&mc.lock);
        mc.from = NULL;
        mc.to = NULL;
-       mc.moving_task = NULL;
        spin_unlock(&mc.lock);
+       mc.moving_task = NULL;
+       mc.mm = NULL;
        mem_cgroup_end_move(from);
        memcg_oom_recover(from);
        memcg_oom_recover(to);
@@ -4724,12 +4729,21 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
                        return 0;
                /* We move charges only when we move a owner of the mm */
                if (mm->owner == p) {
+                       /*
+                        * We do all the move charge works under one mmap_sem to
+                        * avoid deadlock with down_write(&mmap_sem)
+                        * -> try_charge() -> if (mc.moving_task) -> sleep.
+                        */
+                       down_read(&mm->mmap_sem);
+
                        VM_BUG_ON(mc.from);
                        VM_BUG_ON(mc.to);
                        VM_BUG_ON(mc.precharge);
                        VM_BUG_ON(mc.moved_charge);
                        VM_BUG_ON(mc.moved_swap);
                        VM_BUG_ON(mc.moving_task);
+                       VM_BUG_ON(mc.mm);
+
                        mem_cgroup_start_move(from);
                        spin_lock(&mc.lock);
                        mc.from = from;
@@ -4737,14 +4751,16 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
                        mc.precharge = 0;
                        mc.moved_charge = 0;
                        mc.moved_swap = 0;
-                       mc.moving_task = current;
                        spin_unlock(&mc.lock);
+                       mc.moving_task = current;
+                       mc.mm = mm;
 
                        ret = mem_cgroup_precharge_mc(mm);
                        if (ret)
                                mem_cgroup_clear_mc();
-               }
-               mmput(mm);
+                       /* We call up_read() and mmput() in clear_mc(). */
+               } else
+                       mmput(mm);
        }
        return ret;
 }
@@ -4832,7 +4848,7 @@ static void mem_cgroup_move_charge(struct mm_struct *mm)
        struct vm_area_struct *vma;
 
        lru_add_drain_all();
-       down_read(&mm->mmap_sem);
+       /* We've already held the mmap_sem */
        for (vma = mm->mmap; vma; vma = vma->vm_next) {
                int ret;
                struct mm_walk mem_cgroup_move_charge_walk = {
@@ -4851,7 +4867,6 @@ static void mem_cgroup_move_charge(struct mm_struct *mm)
                         */
                        break;
        }
-       up_read(&mm->mmap_sem);
 }
 
 static void mem_cgroup_move_task(struct cgroup_subsys *ss,
@@ -4860,17 +4875,11 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
                                struct task_struct *p,
                                bool threadgroup)
 {
-       struct mm_struct *mm;
-
-       if (!mc.to)
+       if (!mc.mm)
                /* no need to move charge */
                return;
 
-       mm = get_task_mm(p);
-       if (mm) {
-               mem_cgroup_move_charge(mm);
-               mmput(mm);
-       }
+       mem_cgroup_move_charge(mc.mm);
        mem_cgroup_clear_mc();
 }
 #else  /* !CONFIG_MMU */