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[PATCH 3/8] cgroup: add function to get task's cgroup

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  [PATCH 3/8] cgroup: add function to get task's cgroup serge@hallyn.com - 2015-11-16 21:00 +0100
    Re: [PATCH 3/8] cgroup: add function to get task's cgroup Tejun Heo <tj@kernel.org> - 2015-11-24 17:30 +0100
      Re: [PATCH 3/8] cgroup: add function to get task's cgroup Tejun Heo <tj@kernel.org> - 2015-11-24 18:00 +0100

#1270527 — [PATCH 3/8] cgroup: add function to get task's cgroup

From: Aditya Kali <adityakali@google.com>

get_task_cgroup() returns the (reference counted) cgroup of the
given task.

Signed-off-by: Aditya Kali <adityakali@google.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
---
 include/linux/cgroup.h |    1 +
 kernel/cgroup.c        |   25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 22e3754..29f0b02 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -326,6 +326,7 @@ static inline bool css_tryget_online(struct cgroup_subsys_state *css)
 		return percpu_ref_tryget_live(&css->refcnt);
 	return true;
 }
+struct cgroup *get_task_cgroup(struct task_struct *task);
 
 /**
  * css_put - put a css reference
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f1603c1..e29c346 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2210,6 +2210,31 @@ char *task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
 }
 EXPORT_SYMBOL_GPL(task_cgroup_path);
 
+/*
+ * get_task_cgroup - returns the cgroup of the task in the default cgroup
+ * hierarchy.
+ *
+ * @task: target task
+ * This function returns the @task's cgroup on the default cgroup hierarchy. The
+ * returned cgroup has its reference incremented (by calling cgroup_get()). So
+ * the caller must cgroup_put() the obtained reference once it is done with it.
+ */
+struct cgroup *get_task_cgroup(struct task_struct *task)
+{
+	struct cgroup *cgrp;
+
+	mutex_lock(&cgroup_mutex);
+	spin_lock_bh(&css_set_lock);
+
+	cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
+	cgroup_get(cgrp);
+
+	spin_unlock_bh(&css_set_lock);
+	mutex_unlock(&cgroup_mutex);
+	return cgrp;
+}
+EXPORT_SYMBOL_GPL(get_task_cgroup);
+
 /* used to track tasks and other necessary states during migration */
 struct cgroup_taskset {
 	/* the src and dst cset list running through cset->mg_node */
-- 
1.7.9.5

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#1276612

Hello,

On Mon, Nov 16, 2015 at 01:51:40PM -0600, serge@hallyn.com wrote:
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 22e3754..29f0b02 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -326,6 +326,7 @@ static inline bool css_tryget_online(struct cgroup_subsys_state *css)
>  		return percpu_ref_tryget_live(&css->refcnt);
>  	return true;
>  }
> +struct cgroup *get_task_cgroup(struct task_struct *task);

Please move this where other prototypes are.

> +/*
> + * get_task_cgroup - returns the cgroup of the task in the default cgroup
> + * hierarchy.
> + *
> + * @task: target task
> + * This function returns the @task's cgroup on the default cgroup hierarchy. The
> + * returned cgroup has its reference incremented (by calling cgroup_get()). So
> + * the caller must cgroup_put() the obtained reference once it is done with it.
> + */
> +struct cgroup *get_task_cgroup(struct task_struct *task)
> +{
> +	struct cgroup *cgrp;
> +
> +	mutex_lock(&cgroup_mutex);
> +	spin_lock_bh(&css_set_lock);
> +
> +	cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
> +	cgroup_get(cgrp);
> +
> +	spin_unlock_bh(&css_set_lock);
> +	mutex_unlock(&cgroup_mutex);
> +	return cgrp;
> +}
> +EXPORT_SYMBOL_GPL(get_task_cgroup);

So, exposing cgroup_mutex this way can lead to ugly lock dependency
issues as cgroup_mutex is expected to be outside of pretty much
everything.  task_cgroup_path() does it but it has no users (should
prolly removed) and cgroup_attach_task_all() is pretty specific.

Hmmm... cc'ing Li (btw, please cc him and Johannes from the next
posting).  Li, I don't think cset_cgroup_from_root() really needs
cgroup_mutex.  css_set_lock seems to be enough.  What do you think?

Thanks.

-- 
tejun
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#1276656

On Tue, Nov 24, 2015 at 11:27:28AM -0500, Tejun Heo wrote:
> > +struct cgroup *get_task_cgroup(struct task_struct *task)

Umm... is this function even used?

-- 
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