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From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Davide Libenzi <davidel@xmailserver.org>,
       Jan Engelhardt <jengelh@linux01.gwdg.de>, Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Robin Holt <holt@sgi.com>, Roland McGrath <roland@redhat.com>,
       "Serge E. Hallyn" <serge@hallyn.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kthread: Don't depend on work queues
References: <20070410185133.GA104@tv-sign.ru>
	<m13b37v517.fsf@ebiederm.dsl.xmission.com>
	<20070411120431.GB165@tv-sign.ru>
Date: Wed, 11 Apr 2007 10:25:48 -0600
In-Reply-To: <20070411120431.GB165@tv-sign.ru> (Oleg Nesterov's message of
	"Wed, 11 Apr 2007 16:04:31 +0400")
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Oleg Nesterov <oleg@tv-sign.ru> writes:

> On 04/10, Eric W. Biederman wrote:
>> 
>> +int kthreadd(void *unused)
>> +{
>> +	/* Setup a clean context for our children to inherit. */
>> +	kthreadd_setup();
>> +
>> +	current->flags |= PF_NOFREEZE;
>> +
>> +	for (;;) {
>> +		wait_event(kthread_create_work,
>> +			   !list_empty(&kthread_create_list));
>> +
>> +		spin_lock(&kthread_create_lock);
>> +		while (!list_empty(&kthread_create_list)) {
>
> Do we need to check the condition under lock? We can miss an event,
> but then it will be noticed by wait_event() above.

We need to be certain there is something on the list before we remove
it.  Otherwise we will start dereferencing bad pointers.

> IOW,
>
> 	for (;;) {
> 		wait_event(kthread_create_work,
> 			   !list_empty(&kthread_create_list));
>
> 		while (!list_empty(&kthread_create_list)) {
> 			struct kthread_create_info *create;
>
> 			spin_lock(&kthread_create_lock);
> 			create = list_entry(kthread_create_list.next,
> 					    struct kthread_create_info, list);
> 			list_del_init(&create->list);
> 			spin_unlock(&kthread_create_lock);
>
> 			create_kthread(create);
> 		}
> 	}

I guess since we are the only process to ever remove things from the
list that would be safe.

Eric