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    SubjectRe: [PATCH][BUGFIX] cgroups: more safe tasklist locking in cgroup_attach_proc
    On Tue, Aug 16, 2011 at 12:50:06AM +0200, Frederic Weisbecker wrote:
    > On Mon, Aug 15, 2011 at 08:49:57PM +0200, Oleg Nesterov wrote:
    > > On 07/29, Ben Blum wrote:
    > > >
    > > > According to this thread - https://lkml.org/lkml/2011/7/27/243 - RCU is
    > > > not sufficient to guarantee the tasklist is stable w.r.t. de_thread and
    > > > exit. Taking tasklist_lock for reading, instead of rcu_read_lock,
    > > > ensures proper exclusion.
    > >
    > > Yes.
    > >
    > > So far I still think we should fix while_each_thread() so that it works
    > > under rcu_read_lock() "as exepected", I'll try to think more.
    > >
    > > But whatever we do with while_each_thread(), this can't help
    > > cgroup_attach_proc(), it needs the locking.
    > >
    > > > - rcu_read_lock();
    > > > + read_lock(&tasklist_lock);
    > > > if (!thread_group_leader(leader)) {
    > >
    > > Agreed, this should work.
    > >
    > > But can't we avoid the global list? thread_group_leader() or not, we do
    > > not really care. We only need to ensure we can safely find all threads.
    > >
    > > How about the patch below?
    > >
    > >
    > > With or without this/your patch this leader can die right after we
    > > drop the lock. ss->can_attach(leader) and ss->attach(leader) look
    > > suspicious. If a sub-thread execs, this task_struct has nothing to
    > > do with the threadgroup.
    > >
    > >
    > >
    > > Also. This is off-topic, but... Why cgroup_attach_proc() and
    > > cgroup_attach_task() do ->attach_task() + cgroup_task_migrate()
    > > in the different order? cgroup_attach_proc() looks wrong even
    > > if currently doesn't matter.
    >
    > Right. As we concluded in our off-list discussion, if there
    > is no strong reason for that, I'm going to fix that in my task
    > counter patchset because there it really matters. If we can't
    > migrate the thread because it has already exited, we really
    > don't want to call ->attach_task() but rather cancel_attach_task().
    >
    > Thanks.
    >

    Yes. Um, this must have been a mistake on my part. The lines of code
    should be the other way around. It should be done in a separate bugfix
    patch, though, so it goes through faster...

    -- Ben


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