Messages in this thread |  | | From | (Eric W. Biederman) | Subject | Re: [PATCH 1/3] procfs: Export next_tgid(), move it to kernel/pid.c | Date | Mon, 30 Jan 2012 17:51:20 -0800 |
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Anton Vorontsov <anton.vorontsov@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 sufficient.
Eric
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