cgroup: Implement css_task_iter_skip()

When a task is moved out of a cset, task iterators pointing to the
task are advanced using the normal css_task_iter_advance() call.  This
is fine but we'll be tracking dying tasks on csets and thus moving
tasks from cset->tasks to (to be added) cset->dying_tasks.  When we
remove a task from cset->tasks, if we advance the iterators, they may
move over to the next cset before we had the chance to add the task
back on the dying list, which can allow the task to escape iteration.

This patch separates out skipping from advancing.  Skipping only moves
the affected iterators to the next pointer rather than fully advancing
it and the following advancing will recognize that the cursor has
already been moved forward and do the rest of advancing.  This ensures
that when a task moves from one list to another in its cset, as long
as it moves in the right direction, it's always visible to iteration.

This doesn't cause any visible behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index a7e4611e20c8..05ed2a209e74 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -43,6 +43,9 @@
 /* walk all threaded css_sets in the domain */
 #define CSS_TASK_ITER_THREADED		(1U << 1)
 
+/* internal flags */
+#define CSS_TASK_ITER_SKIPPED		(1U << 16)
+
 /* a css_task_iter should be treated as an opaque object */
 struct css_task_iter {
 	struct cgroup_subsys		*ss;