linux-kernel - Re: [PATCH 1/3] cgroup: remove tasklist_lock from cgroup_attach

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Date:	Fri, 23 Dec 2011 03:41:24 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Mandeep Singh Baines <msb@...omium.org>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [PATCH 1/3] cgroup: remove tasklist_lock from cgroup_attach_proc

On Fri, Dec 23, 2011 at 10:40:28AM +0800, Li Zefan wrote:
> >> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> >> index 1042b3c..032139d 100644
> >> --- a/kernel/cgroup.c
> >> +++ b/kernel/cgroup.c
> >> @@ -2102,21 +2102,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
> >>  	if (retval)
> >>  		goto out_free_group_list;
> >>  
> >> -	/* prevent changes to the threadgroup list while we take a snapshot. */
> >> -	read_lock(&tasklist_lock);
> >> -	if (!thread_group_leader(leader)) {
> >> -		/*
> >> -		 * a race with de_thread from another thread's exec() may strip
> >> -		 * us of our leadership, making while_each_thread unsafe to use
> >> -		 * on this task. if this happens, there is no choice but to
> >> -		 * throw this task away and try again (from cgroup_procs_write);
> >> -		 * this is "double-double-toil-and-trouble-check locking".
> >> -		 */
> >> -		read_unlock(&tasklist_lock);
> >> -		retval = -EAGAIN;
> >> -		goto out_free_group_list;
> >> -	}
> >> -
> >>  	tsk = leader;
> >>  	i = 0;
> >>  	do {
> >> @@ -2145,7 +2130,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
> >>  	group_size = i;
> >>  	tset.tc_array = group;
> >>  	tset.tc_array_len = group_size;
> >> -	read_unlock(&tasklist_lock);
> > 
> > You still need rcu_read_lock()/rcu_read_unlock() around
> > 	do {
> > 
> > 	} while_each_thread()
> > 
> > because threadgroup_lock() doesn't lock the part that remove a thread from
> > its group on exit.
> > 
> 
> and inside rcu critical section, you can't call kmalloc(GFP_KERNEL)!!

Good point. Well it's still worth replacing tasklist_lock by rcu though :)
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