[PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c
Eric W. Biederman
ebiederm at xmission.com
Tue Jan 31 01:51:20 UTC 2012
Anton Vorontsov <anton.vorontsov at linaro.org> 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
More information about the linaro-kernel