linux-kernel - Re: [PATCH] cgroup: fix lockdep warning for event

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Date:	Wed, 28 Nov 2012 13:33:11 -0800
From:	Greg Thelen <gthelen@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Li Zefan <lizefan@...wei.com>,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Aaron Durbin <adurbin@...gle.com>
Subject: Re: [PATCH] cgroup: fix lockdep warning for event_control

On Wed, Nov 28 2012, Tejun Heo wrote:

> Hello, Greg.
>
> On Wed, Nov 28, 2012 at 12:15:42PM -0800, Greg Thelen wrote:
>> @@ -4276,6 +4276,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
>>  	DEFINE_WAIT(wait);
>>  	struct cgroup_event *event, *tmp;
>>  	struct cgroup_subsys *ss;
>> +	struct list_head tmp_list;
>
> LIST_HEAD(tmp_list);
>
>>  
>>  	lockdep_assert_held(&d->d_inode->i_mutex);
>>  	lockdep_assert_held(&cgroup_mutex);
>> @@ -4330,16 +4331,20 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
>>  	/*
>>  	 * Unregister events and notify userspace.
>>  	 * Notify userspace about cgroup removing only after rmdir of cgroup
>> -	 * directory to avoid race between userspace and kernelspace
>> +	 * directory to avoid race between userspace and kernelspace. Use
>> +	 * a temporary list to avoid a deadlock with cgroup_event_wake(). Since
>> +	 * cgroup_event_wake() is called with the wait queue head locked,
>> +	 * remove_wait_queue() cannot be called while holding event_list_lock.
>>  	 */
>>  	spin_lock(&cgrp->event_list_lock);
>> -	list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
>> +	list_replace_init(&cgrp->event_list, &tmp_list);
>
> list_splice_init();
>
> would be more conventional, I think.

I used list_replace_init() because it avoided unnecessary init of
tmp_list.  But I have no objection to list_splice_init().  I'll repost
with list_splice_init().

>> +	spin_unlock(&cgrp->event_list_lock);
>> +	list_for_each_entry_safe(event, tmp, &tmp_list, list) {
>>  		list_del(&event->list);
>
> Maybe convert this to list_del_init() while at it?

I assume this isn't technically required, but defensive.  Seems like we
should also convert the list_del(&event->list) in cgroup_event_wake()
for consistency.  I'll send these in separate patch.

>>  		remove_wait_queue(event->wqh, &event->wait);
>>  		eventfd_signal(event->eventfd, 1);
>>  		schedule_work(&event->remove);
>>  	}
>> -	spin_unlock(&cgrp->event_list_lock);
>
> Thanks.
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