From: Oleg Nesterov <oleg@redhat.com>
To: Mandeep Singh Baines <msb@chromium.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
Li Zefan <lizf@cn.fujitsu.com>, Tejun Heo <tj@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Containers <containers@lists.linux-foundation.org>,
Cgroups <cgroups@vger.kernel.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
Paul Menage <paul@paulmenage.org>,
Andrew Morton <akpm@linux-foundation.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking
Date: Tue, 20 Mar 2012 20:34:14 +0100 [thread overview]
Message-ID: <20120320193414.GA21277@redhat.com> (raw)
In-Reply-To: <20120119154522.GA14058@redhat.com>
OK, finally we should do something with this problem ;)
On 01/19, Oleg Nesterov wrote:
>
> I'll try to investigate if we can remove
>
> leader->group_leader = tsk;
>
> from de_thread(). In fact I already thought about this change a long
> ago without any connection to while_each_thread(). This assignment
> looks "assymetrical" compared to other threads we kill. But we did
> have a reason (reasons?). Hopefully, the only really important reason
> was already removed by 087806b1.
On the second thought, I think we should not do this.
For example, do_prlimit() assumes that tsk->group_leader is correct
under tasklist_lock.
OK, lets return to the thread_group_leader() check. To ensure we do
not visit the same thread twice we can check 'g', not 't'.
This is what I am going to send, after I re-check once again...
I have the problem with the changelog ;) Somehow it should explain
that while_each_thread_rcu(g, t) can't race with do_group_exit().
I think it can't, list_del_rcu(leader->thread_group) happens when
this entry is already "empty", it should be the last thread in group.
If the non-leader thread goes away from the least, we still have
the "path" to reach the leader. But this is not easy to explain.
As for the barrier. If de_thread() changes the leader it drops
and re-acquires tasklist_lock (this implies mb) after it changes
old_leader->exit_signal (used in thread_group_leader) and before
__unhash_process() which does list_del_rcu().
This means that if while_each_thread() sees the result of
list_del_rcu(old_leader) it must also see that
thread_group_leader(old_leader) != T.
What do you think? Do you see any problems?
Oleg.
---
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 7d379a6..f169bfd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2323,9 +2323,24 @@ extern bool current_is_single_threaded(void);
#define do_each_thread(g, t) \
for (g = t = &init_task ; (g = t = next_task(g)) != &init_task ; ) do
+/*
+ * needs tasklist_lock or ->siglock, or rcu if the caller ensures
+ * that 'g' can't exit or exec.
+ */
#define while_each_thread(g, t) \
while ((t = next_thread(t)) != g)
+/*
+ * rcu-safe, but should start at ->group_leader.
+ * thread_group_leader(g) protects against the race with exec which
+ * removes the leader from list.
+ * smp_rmb() pairs with implicit mb() implied by unlock + lock in
+ * de_thread()->release_task() path.
+ */
+#define while_each_thread_rcu(g, t) \
+ while ((t = next_thread(t)) != g && \
+ ({ smp_rmb(); thread_group_leader(g); }))
+
static inline int get_nr_threads(struct task_struct *tsk)
{
return tsk->signal->nr_threads;
next prev parent reply other threads:[~2012-03-20 19:41 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-21 3:43 Q: cgroup: Questions about possible issues in cgroup locking Frederic Weisbecker
2011-12-21 13:08 ` Oleg Nesterov
2011-12-21 17:56 ` Frederic Weisbecker
2011-12-21 19:01 ` Mandeep Singh Baines
2011-12-21 19:08 ` Frederic Weisbecker
2011-12-21 19:24 ` Mandeep Singh Baines
2011-12-21 20:04 ` Frederic Weisbecker
2011-12-22 15:30 ` Oleg Nesterov
2012-01-04 19:36 ` Mandeep Singh Baines
2012-01-06 15:23 ` Oleg Nesterov
2012-01-06 18:25 ` Mandeep Singh Baines
2012-01-11 16:07 ` Oleg Nesterov
2012-01-12 0:31 ` Mandeep Singh Baines
2012-01-12 17:07 ` Oleg Nesterov
2012-01-12 17:57 ` Mandeep Singh Baines
2012-01-13 15:20 ` Oleg Nesterov
2012-01-13 18:27 ` Mandeep Singh Baines
2012-01-14 17:36 ` Oleg Nesterov
2012-01-18 23:17 ` Mandeep Singh Baines
2012-01-19 15:45 ` Oleg Nesterov
2012-01-19 18:18 ` Mandeep Singh Baines
2012-01-20 15:06 ` Oleg Nesterov
2012-03-20 19:34 ` Oleg Nesterov [this message]
2012-03-21 18:59 ` Mandeep Singh Baines
2012-03-23 17:51 ` Oleg Nesterov
2011-12-21 17:59 ` Frederic Weisbecker
2011-12-21 18:11 ` Oleg Nesterov
2011-12-21 18:23 ` Frederic Weisbecker
2012-02-01 16:28 ` Frederic Weisbecker
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20120320193414.GA21277@redhat.com \
--to=oleg@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=cgroups@vger.kernel.org \
--cc=containers@lists.linux-foundation.org \
--cc=fweisbec@gmail.com \
--cc=kamezawa.hiroyu@jp.fujitsu.com \
--cc=linux-kernel@vger.kernel.org \
--cc=lizf@cn.fujitsu.com \
--cc=msb@chromium.org \
--cc=paul@paulmenage.org \
--cc=paulmck@linux.vnet.ibm.com \
--cc=tj@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).