linux-kernel - Re: [PATCH 3/5] Container Freezer: Implement freezer cgroup subsystem

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 04 Nov 2008 14:12:05 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	Matt Helsley <matthltc@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-pm@...ts.linux-foundation.org,
	Cedric Le Goater <clg@...ibm.com>,
	"Serge E. Hallyn" <serue@...ibm.com>
Subject: Re: [PATCH 3/5] Container Freezer: Implement freezer cgroup subsystem

Paul Menage wrote:
> On Mon, Aug 11, 2008 at 3:53 PM, Matt Helsley <matthltc@...ibm.com> wrote:
>> +}
>> +
>> +static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
>> +{
>> +       struct freezer *freezer;
>> +
>> +       task_lock(task);
>> +       freezer = task_freezer(task);
>> +       task_unlock(task);
>> +
>> +       BUG_ON(freezer->state == STATE_FROZEN);
>> +       spin_lock_irq(&freezer->lock);
>> +       /* Locking avoids race with FREEZING -> RUNNING transitions. */
>> +       if (freezer->state == STATE_FREEZING)
>> +               freeze_task(task, true);
>> +       spin_unlock_irq(&freezer->lock);
>> +}
> 
> Sorry for such a delayed response to this patch, but I just noticed
> (in mainline now)
> the change to move the task_lock() to only encompass the
> task_freezer() call.
> 
> That results in absolutely zero protection from the task_lock() - as
> soon as you drop it, then in theory the current task could move cgroup
> and the old freezer structure be freed.
> 
> Having said that, I think that in this case any locking my be
> unnecessary since task isn't on the tasklist yet, so can't be selected
> to move cgroups. (Although this does make me wonder whether
> cpuset.c:move_member_tasks_to_cpuset() can fail silently if it races
> with a fork).
> 
> On top of that, for a system that configures in the cgroup freezer
> system but doesn't ever use it, every task is in the same freezer
> cgroup (the root cgroup) and task_freezer(task)->lock becomes a global
> spinlock. I think this would certainly show up on some benchmarks
> although I don't know how bad it would be in a practical sense. But it
> might be worth considering making using of the cgroup bind() callback
> to track whether or not the freezer subsystem is in use, and
> short-circuiting this in freezer_fork() without any locking if you're
> not active.
> 

I think another reasonable and easier way is to disable writing freezer.state
of top cgroup, so we can skip checks in freezer_fork() for tasks in top cgroup.

--
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/