Re: [PATCH 2/2] cgroup: Walk task list under tasklist_lock incgroup_enable_task_cg_list

[Frederic Weisbecker]

On Tue, Feb 21, 2012 at 05:19:34PM -0800, Paul E. McKenney wrote:
> 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@xxxxxxxxx) 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@xxxxxxxxx>
> > > > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > > > Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
> > > > Cc: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
> > > 
> > > Reviewed-by: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
> > > 
> > > Sorry for being late. My feedback is really just comments.
> > > 
> > > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > ---
> > > >  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

Cool that confirm what I was thinking. Thanks for the clarification!
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