Messages in this thread | | | Date | Fri, 17 Feb 2012 13:41:21 +0800 | From | Li Zefan <> | Subject | Re: [RFC][PATCH 0/2] cgroup: Fix some races against css_set task links |
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sorry for the delayed reply. I had been off for quite a few days.
Frederic Weisbecker wrote: > Hi, > > This is trying to fix some races in the way to link all the tasks to > the css_set links. I hope some people can have a look at this, especially > as I'm definetly not an SMP ordering expert.
me neither.
> > To give you the big picture, as long as nobody never calls > cgroup_iter_start(), we don't link the tasks to their css_set (this 'link' > is a simple list_add()). > > But once somebody calls cgroup_iter_start(), we call > cgroup_enable_task_cg_lists() that grossly does this: > > cgroup_enable_task_cg_lists() { > use_task_set_css_links = 1; > do_each_thread(g, p) { > link p to css_set > } while_each_thread(g, p); > } > > But this links only existing tasks, we also need to link all the tasks > that will be created later, this is what does cgroup_post_fork(): > > cgroup_post_fork() { > if (use_task_set_css_links) > link p to css_set > } > > So we have some races here: > > - cgroup_enable_task_cg_lists() iterates over the tasklist > without protection. The comments are advertizing we are using RCU > but we don't. And in fact RCU doesn't yet protect against > while_each_thread(). > > - Moreover with RCU there is a risk that we iterate the tasklist but > we don't immediately see all the last updates that happened. For > example if a task forks and passes cgroup_post_fork() while > use_task_set_css_links = 0 then another CPU calling > cgroup_enable_task_cg_list() can miss the new child while walking the > tasklist with RCU as it doesn't appear immediately. > > - There is no ordering constraint on use_task_set_css_links read/write > against the tasklist traversal and modification. cgroup_post_fork() > may deal with a stale value. > > The second patch of the series is a proposal to fix the three above > points. Tell me what you think. >
The patch looks good to me.
As cgroup_enable_task_cg_lists() is an one-off function, won't be called more than once, so there's little chance it can happen in reality, so should be ok to queue it for 3.4.
> Thanks. > > Frederic Weisbecker (2): > cgroup: Remove wrong comment on cgroup_enable_task_cg_list() > cgroup: Walk task list under tasklist_lock in > cgroup_enable_task_cg_list >
for both patches
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
> kernel/cgroup.c | 23 ++++++++++++++++++++--- > 1 files changed, 20 insertions(+), 3 deletions(-) >
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