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    SubjectRe: [PATCH 2/2] cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list
    On Wed, Feb 22, 2012 at 01:55:28AM +0100, Frederic Weisbecker wrote:
    > On Tue, Feb 21, 2012 at 02:23:43PM -0800, Mandeep Singh Baines wrote:
    > > Frederic Weisbecker (fweisbec@gmail.com) wrote:
    > > > Walking through the tasklist in cgroup_enable_task_cg_list() inside
    > > > an RCU read side critical section is not enough because:
    > > >
    > > > - RCU is not (yet) safe against while_each_thread()
    > > >
    > > > - If we use only RCU, a forking task that has passed cgroup_post_fork()
    > > > without seeing use_task_css_set_links == 1 is not guaranteed to have
    > > > its child immediately visible in the tasklist if we walk through it
    > > > remotely with RCU. In this case it will be missing in its css_set's
    > > > task list.
    > > >
    > > > Thus we need to traverse the list (unfortunately) under the
    > > > tasklist_lock. It makes us safe against while_each_thread() and also
    > > > make sure we see all forked task that have been added to the tasklist.
    > > >
    > > > As a secondary effect, reading and writing use_task_css_set_links are
    > > > now well ordered against tasklist traversing and modification. The new
    > > > layout is:
    > > >
    > > > CPU 0 CPU 1
    > > >
    > > > use_task_css_set_links = 1 write_lock(tasklist_lock)
    > > > read_lock(tasklist_lock) add task to tasklist
    > > > do_each_thread() { write_unlock(tasklist_lock)
    > > > add thread to css set links if (use_task_css_set_links)
    > > > } while_each_thread() add thread to css set links
    > > > read_unlock(tasklist_lock)
    > > >
    > > > If CPU 0 traverse the list after the task has been added to the tasklist
    > > > then it is correctly added to the css set links. OTOH if CPU 0 traverse
    > > > the tasklist before the new task had the opportunity to be added to the
    > > > tasklist because it was too early in the fork process, then CPU 1
    > > > catches up and add the task to the css set links after it added the task
    > > > to the tasklist. The right value of use_task_css_set_links is guaranteed
    > > > to be visible from CPU 1 due to the LOCK/UNLOCK implicit barrier properties:
    > > > the read_unlock on CPU 0 makes the write on use_task_css_set_links happening
    > > > and the write_lock on CPU 1 make the read of use_task_css_set_links that comes
    > > > afterward to return the correct value.
    > > >
    > > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > > > Cc: Tejun Heo <tj@kernel.org>
    > > > Cc: Li Zefan <lizf@cn.fujitsu.com>
    > > > Cc: Mandeep Singh Baines <msb@chromium.org>
    > >
    > > Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
    > >
    > > Sorry for being late. My feedback is really just comments.
    > >
    > > > Cc: Oleg Nesterov <oleg@redhat.com>
    > > > Cc: Andrew Morton <akpm@linux-foundation.org>
    > > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > > ---
    > > > kernel/cgroup.c | 20 ++++++++++++++++++++
    > > > 1 files changed, 20 insertions(+), 0 deletions(-)
    > > >
    > > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c
    > > > index 6e4eb43..c6877fe 100644
    > > > --- a/kernel/cgroup.c
    > > > +++ b/kernel/cgroup.c
    > > > @@ -2707,6 +2707,14 @@ static void cgroup_enable_task_cg_lists(void)
    > > > struct task_struct *p, *g;
    > > > write_lock(&css_set_lock);
    > >
    > > You might want to re-test use_task_css_set_links once you have the lock
    > > in order to avoid an unnecessary do_each_thread()/while_each_thread() in
    > > case you race between reading the value and entering the loop. This is
    > > a potential optimization in a rare case so maybe not worth the LOC.
    >
    > Makes sense. I'll do that in a seperate patch.
    >
    > >
    > > > use_task_css_set_links = 1;
    > > > + /*
    > > > + * We need tasklist_lock because RCU is not safe against
    > > > + * while_each_thread(). Besides, a forking task that has passed
    > > > + * cgroup_post_fork() without seeing use_task_css_set_links = 1
    > > > + * is not guaranteed to have its child immediately visible in the
    > > > + * tasklist if we walk through it with RCU.
    > > > + */
    > >
    > > Maybe add TODO to remove the lock once do_each_thread()/while_each_thread()
    > > is made rcu safe. On a large system, it could take a while to iterate
    > > over every thread in the system. Thats a long time to hold a spinlock.
    > > But it only happens once so probably not that big a deal.
    >
    > I think that even if while_each_thread() was RCU safe, that wouldn't
    > work here.
    >
    > Unless I'm mistaken, we have no guarantee that a remote list_add_rcu()
    > is immediately visible by the local CPU if it walks the list under
    > rcu_read_lock() only.

    Indeed, the guarantee is instead that -if- a reader encounters a newly
    added list element, then that reader will see any initialization of that
    list element carried out prior to the list_add_rcu().

    Memory barriers are about ordering, not about making memory writes
    visible faster.

    Thanx, Paul

    > Consider that ordering scenario:
    >
    > CPU 0 CPU 1
    > --------------- --------------
    >
    > fork() {
    > write_lock(tasklist_lock);
    > add child to tasklist
    > write_unlock(tasklist_lock);
    > cgroup_post_fork()
    > }
    >
    > cgroup_enable_task_cg_lists() {
    > rcu_read_lock();
    > do_each_thread() {
    > ..... <-- find child ?
    > } while_each_thread()
    > rcu_read_unlock()
    >
    >
    > We have no guarantee here that the write on CPU 0 will be visible
    > in time to CPU 1.
    >
    > But may be I misunderstood the ordering and committing guarantees with RCU.
    > Perhaps Paul can confirm or correct me.
    >
    > Paul?
    >



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