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Date: Wed, 22 Feb 2012 01:55:28 +0100 From: Frederic Weisbecker <fweisbec@...il.com> To: Mandeep Singh Baines <msb@...omium.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>, LKML <linux-kernel@...r.kernel.org>, Oleg Nesterov <oleg@...hat.com>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [PATCH 2/2] cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list On Tue, Feb 21, 2012 at 02:23:43PM -0800, Mandeep Singh Baines wrote: > 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. 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. 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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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