[PATCH] cgroup, docs: add a note about returning EBUSY in some cases

[Roman Gushchin]

Explicitly document EBUSY returned by writing into cgroup.procs
if controllers are enabled; and writing into cgroup.subtree_control
if there are attached processes.

The return code might be slightly surprising, and because there is
nothing obviously better, let's document it at least.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: kernel-team@xxxxxx
Cc: cgroups@xxxxxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 Documentation/cgroup-v2.txt | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 74cdeaed9f7a..57302f88a4ad 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -799,6 +799,9 @@ All cgroup core files are prefixed with "cgroup."
 	When delegating a sub-hierarchy, write access to this file
 	should be granted along with the containing directory.
 
+	If the target cgroup has enabled controllers, writing to this
+	file will fail with EBUSY.
+
 	In a threaded cgroup, reading this file fails with EOPNOTSUPP
 	as all the processes belong to the thread root.  Writing is
 	supported and moves every thread of the process to the cgroup.
@@ -850,6 +853,9 @@ All cgroup core files are prefixed with "cgroup."
 	the last one is effective.  When multiple enable and disable
 	operations are specified, either all succeed or all fail.
 
+	If the cgroup has attached tasks, writing to this file will
+	fail with EBUSY.
+
   cgroup.events
 	A read-only flat-keyed file which exists on non-root cgroups.
 	The following entries are defined.  Unless specified
-- 
2.14.3