Date:	Tue, 31 Jan 2012 20:37:00 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	San Mehat <san@...gle.com>, Colin Cross <ccross@...roid.com>,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c

Anton Vorontsov <anton.vorontsov@...aro.org> writes:

> On Mon, Jan 30, 2012 at 05:51:20PM -0800, Eric W. Biederman wrote:
> [...]
>> > Yes, in LMK driver we don't need to be accurate. I probably could use
>> > rcu_read_lock, but the plan was in not holding any global locks (in
>> > this case the rcu) at all, instead I'd like to hold just a reference
>> > of the task, which the driver is analyzing at this time. Once we decide
>> > (to kill or not to kill the task), we either send a signal (and drop
>> > the reference) or just drop the reference.
>> 
>> rcu_read_lock unless it is implemented wrong is free from a lock
>> perspective.  rcu_read_lock only touches local state.
>> 
>> >From the look of your loop it already does a walk through the entire
>> process list so it looks to me like playing games with get_task_struct
>> and put_task_struct are going to be much more expensive.
>> 
>> proc grabs task references because we can't hold the rcu_read_lock
>> over a copy_to_user because that is a sleeping function.
>> 
>> You don't call anything that sleeps so rcu_read_lock should be
>> sufficient.
>
> I'll just repeat what I told to Paul E. McKenney:
>
> [...] the locking part wasn't my concern at all. As I said before,
> LMK (low memory killer) itself is not important, and we don't care
> about its overhead, unless it blocks another kernel activity --
> which is my main concern.
>
> So, reader part is not interesting in sense of overhead or
> efficiency.
>
> The interesting questions are:
>
> 1. Can the kernel create processes while LMK traverses the list?
> 2. Can the kernel free processes while LMK traverses the list?
>
> Looking into kernel/fork.c:copy_process(), it does this:
>
> - Takes a write lock on tasklist_lock;
> - Uses list_add_tail_rcu() to add a task.
>
> So, with current LMK driver (it grabs the tasklist_lock), it seems
> that the kernel won't able to create processes while LMK traverse the
> tasks.
>
> Looking into kernel/exit.c:release_task(), it does this:
>
> - Takes a write lock on tasklist_lock;
> - Deletes the task from the list using list_del_rcu()
> - Releases tasklist_lock;
> - Issues call_rcu(&p->rcu, delayed_put_task_struct), which
>   then actually completely frees the task;
>
> So, with the current LMK driver, kernel won't able to release
> processes while LMK traverse the processes, because LMK takes
> the tasklist_lock.
>
> By using rcu_read_lock() we would solve "1.", i.e. kernel will able
> to create processes, but we still won't able to free processes (well,
> for the most part we will, except that we'll only free memory after
> LMK finishes its traverse).


Correct. We will only free the task_struct after you release the
rcu_read_lock. Many of the other resources are freed before the
task_struct.  So most of the memory of a process should be freeable
even with the rcu_read_lock held.

Eric
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