* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-06-01 18:27:20]:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>
> Now, for checking a memcg is under task-account-moving, we do css_tryget()
> against mc.to and mc.from. But this ust complicates things. This patch
> makes the check easier. (And I doubt the current code has some races..)
>
> This patch adds a spinlock to move_charge_struct and guard modification
> of mc.to and mc.from. By this, we don't have to think about complicated
> races around this not-critical path.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
> mm/memcontrol.c | 48 ++++++++++++++++++++++++++++--------------------
> 1 file changed, 28 insertions(+), 20 deletions(-)
>
> Index: mmotm-2.6.34-May21/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.34-May21.orig/mm/memcontrol.c
> +++ mmotm-2.6.34-May21/mm/memcontrol.c
> @@ -268,6 +268,7 @@ 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 */
> struct mem_cgroup *from;
> struct mem_cgroup *to;
> unsigned long precharge;
> @@ -276,6 +277,7 @@ static struct move_charge_struct {
> struct task_struct *moving_task; /* a task moving charges */
> wait_queue_head_t waitq; /* a waitq for other context */
> } mc = {
> + .lock = __SPIN_LOCK_UNLOCKED(mc.lock),
> .waitq = __WAIT_QUEUE_HEAD_INITIALIZER(mc.waitq),
> };
>
> @@ -1076,26 +1078,25 @@ static unsigned int get_swappiness(struc
>
> static bool mem_cgroup_under_move(struct mem_cgroup *mem)
> {
> - struct mem_cgroup *from = mc.from;
> - struct mem_cgroup *to = mc.to;
> + struct mem_cgroup *from;
> + struct mem_cgroup *to;
> bool ret = false;
> -
> - if (from == mem || to == mem)
> - return true;
> -
> - if (!from || !to || !mem->use_hierarchy)
> - return false;
> -
> - rcu_read_lock();
> - if (css_tryget(&from->css)) {
> - ret = css_is_ancestor(&from->css, &mem->css);
> - css_put(&from->css);
> - }
> - if (!ret && css_tryget(&to->css)) {
> - ret = css_is_ancestor(&to->css, &mem->css);
> + /*
> + * Unlike task_move routines, we access mc.to, mc.from not under
> + * mutual execution by cgroup_mutex. Here, we take spinlock instead.
^^^^^
Typo should be exclusion
> + */
> + spin_lock_irq(&mc.lock);
Why do we use the _irq variant here?
--
Three Cheers,
Balbir
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