On Tue, 1 Jun 2010 16:55:29 +0530
Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> * 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
Sure.
>
> > + */
> > + spin_lock_irq(&mc.lock);
>
> Why do we use the _irq variant here?
>
Hmm. I'd like to add preemption_disable() or disable irq here. This spinlock
is held as
cgroup_mutex();
-> mc.lock
Then, I don't want to have a risk for preemption. But yes, logically, disabling irq
isn't necessary. I'll remove _irq() in the next.
Thanks,
-Kame
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