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From: (Eric W. Biederman)
To: Anton Vorontsov <>
Cc: "Oleg Nesterov" <>,
	"Arve Hjønnevåg" <>,
	"KOSAKI Motohiro" <>,
	"Greg Kroah-Hartman" <>,
	"San Mehat" <>, "Colin Cross" <>,,,
Subject: Re: [PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c
Date: Mon, 30 Jan 2012 17:51:20 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Anton Vorontsov's message of "Tue, 31 Jan 2012 00:49:20 +0400")

Anton Vorontsov <> writes:

> On Mon, Jan 30, 2012 at 02:43:12PM +0100, Oleg Nesterov wrote:
>> On 01/30, Anton Vorontsov wrote:
>> >
>> > We'd like to use this function in the android low memory killer driver, so
>> > let's export it.
>> I guess you mean 3/3.
>> If lowmem_shrink() can use next_tid() which is not really accurate,
>> then why tou can't simply change it to use rcu_read_lock ?
> 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


  reply	other threads:[~2012-01-31  1:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-30  1:13 [PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c Anton Vorontsov
2012-01-30  2:22 ` Greg KH
2012-01-30 19:50   ` Anton Vorontsov
2012-01-30  3:26 ` Eric W. Biederman
2012-01-30 19:51   ` Anton Vorontsov
2012-01-30 13:43 ` Oleg Nesterov
2012-01-30 20:49   ` Anton Vorontsov
2012-01-31  1:51     ` Eric W. Biederman [this message]
2012-02-01  4:19       ` Anton Vorontsov
2012-02-01  4:37         ` Eric W. Biederman

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