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Date: Tue, 21 Feb 2012 14:23:43 -0800
From: Mandeep Singh Baines <msb@...omium.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>,
LKML <linux-kernel@...r.kernel.org>,
Mandeep Singh Baines <msb@...omium.org>,
Oleg Nesterov <oleg@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] cgroup: Walk task list under tasklist_lock in
cgroup_enable_task_cg_list
Frederic Weisbecker (fweisbec@...il.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@...il.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Li Zefan <lizf@...fujitsu.com>
> Cc: Mandeep Singh Baines <msb@...omium.org>
Reviewed-by: Mandeep Singh Baines <msb@...omium.org>
Sorry for being late. My feedback is really just comments.
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Paul E. McKenney <paulmck@...ux.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.
> 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.
> + read_lock(&tasklist_lock);
> do_each_thread(g, p) {
> task_lock(p);
> /*
> @@ -2718,6 +2726,7 @@ static void cgroup_enable_task_cg_lists(void)
> list_add(&p->cg_list, &p->cgroups->tasks);
> task_unlock(p);
> } while_each_thread(g, p);
> + read_unlock(&tasklist_lock);
> write_unlock(&css_set_lock);
> }
>
> @@ -4522,6 +4531,17 @@ void cgroup_fork_callbacks(struct task_struct *child)
> */
> void cgroup_post_fork(struct task_struct *child)
> {
> + /*
> + * use_task_css_set_links is set to 1 before we walk the tasklist
> + * under the tasklist_lock and we read it here after we added the child
> + * to the tasklist under the tasklist_lock as well. If the child wasn't
> + * yet in the tasklist when we walked through it from
> + * cgroup_enable_task_cg_lists(), then use_task_css_set_links value
> + * should be visible now due to the paired locking and barriers implied
> + * by LOCK/UNLOCK: it is written before the tasklist_lock unlock
> + * in cgroup_enable_task_cg_lists() and read here after the tasklist_lock
> + * lock on fork.
> + */
> if (use_task_css_set_links) {
> write_lock(&css_set_lock);
> if (list_empty(&child->cg_list)) {
> --
> 1.7.5.4
>
--
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