linux-kernel - Re: [PATCH v4 2/2] cgroups: make procs file writable

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Message-ID: <20100804043849.GC11950@ghc17.ghc.andrew.cmu.edu>
Date:	Wed, 4 Aug 2010 00:38:49 -0400
From:	Ben Blum <bblum@...rew.cmu.edu>
To:	Paul Menage <menage@...gle.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ben Blum <bblum@...rew.cmu.edu>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	ebiederm@...ssion.com, lizf@...fujitsu.com, matthltc@...ibm.com,
	oleg@...hat.com
Subject: Re: [PATCH v4 2/2] cgroups: make procs file writable

On Tue, Aug 03, 2010 at 09:30:00PM -0700, Paul Menage wrote:
> >> --- a/kernel/cpuset.c
> >> +++ b/kernel/cpuset.c
> >> @@ -1404,6 +1404,10 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
> >> ? ? ? ? ? ? ? struct task_struct *c;
> >>
> >> ? ? ? ? ? ? ? rcu_read_lock();
> >> + ? ? ? ? ? ? if (!thread_group_leader(tsk)) {
> >> + ? ? ? ? ? ? ? ? ? ? rcu_read_unlock();
> >> + ? ? ? ? ? ? ? ? ? ? return -EAGAIN;
> >> + ? ? ? ? ? ? }
> 
> Why are you adding this requirement, here and in sched.c? (ns_cgroup.c
> doesn't matter since it's being deleted).
> 
> Paul

It was either this or:

rcu_read_lock();
for_each_subsys(...) {
	can_attach(...);
}
rcu_read_unlock();

Which forces all can_attaches to not sleep. So by dropping
rcu_read_lock(), we allow the possibility of the exec race I described
in my last email, and therefore we have to check each time we re-acquire
rcu_read to iterate thread_group.

Yeah, it is not pretty. I call it "double-double-toil-and-trouble-check
locking". But it is safe.

-- Ben
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