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SubjectRe: [PATCH block/for-next 3/4] cgroup, block: implement task_get_css() and use it in bio_associate_current()
On Mon, Nov 17, 2014 at 04:15:46PM -0500, Tejun Heo wrote:
> bio_associate_current() currently open codes task_css() and
> css_tryget_online() to find and pin $current's blkcg css. Abstract it
> into task_get_css() which is implemented from cgroup side. As a task
> is always associated with an online css for every subsystem except
> while the css_set update is propagating, task_get_css() retries till
> css_tryget_online() succeeds.
>
> This is a cleanup and shouldn't lead to noticeable behavior changes.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Vivek Goyal <vgoyal@redhat.com>

Looks good to me.

Acked-by: Vivek Goyal <vgoyal@redhat.com>

Vivek

> ---
> Hello,
>
> This one can be routed through either cgroup or block tree but I think
> it'd be easier to route this through block as there will be more block
> changes in the area.
>
> Thanks.
>
> block/bio.c | 11 +----------
> include/linux/cgroup.h | 25 +++++++++++++++++++++++++
> 2 files changed, 26 insertions(+), 10 deletions(-)
>
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1997,7 +1997,6 @@ EXPORT_SYMBOL(bioset_create_nobvec);
> int bio_associate_current(struct bio *bio)
> {
> struct io_context *ioc;
> - struct cgroup_subsys_state *css;
>
> if (bio->bi_ioc)
> return -EBUSY;
> @@ -2006,17 +2005,9 @@ int bio_associate_current(struct bio *bi
> if (!ioc)
> return -ENOENT;
>
> - /* acquire active ref on @ioc and associate */
> get_io_context_active(ioc);
> bio->bi_ioc = ioc;
> -
> - /* associate blkcg if exists */
> - rcu_read_lock();
> - css = task_css(current, blkio_cgrp_id);
> - if (css && css_tryget_online(css))
> - bio->bi_css = css;
> - rcu_read_unlock();
> -
> + bio->bi_css = task_get_css(current, blkio_cgrp_id);
> return 0;
> }
>
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -748,6 +748,31 @@ static inline struct cgroup_subsys_state
> }
>
> /**
> + * task_get_css - find and get the css for (task, subsys)
> + * @task: the target task
> + * @subsys_id: the target subsystem ID
> + *
> + * Find the css for the (@task, @subsys_id) combination, increment a
> + * reference on and return it. This function is guaranteed to return a
> + * valid css.
> + */
> +static inline struct cgroup_subsys_state *
> +task_get_css(struct task_struct *task, int subsys_id)
> +{
> + struct cgroup_subsys_state *css;
> +
> + rcu_read_lock();
> + while (true) {
> + css = task_css(task, subsys_id);
> + if (likely(css_tryget_online(css)))
> + break;
> + cpu_relax();
> + }
> + rcu_read_unlock();
> + return css;
> +}
> +
> +/**
> * task_css_is_root - test whether a task belongs to the root css
> * @task: the target task
> * @subsys_id: the target subsystem ID


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