cgroup: Prevent kill_css() from being called more than once

The kill_css() function may be called more than once under the condition
that the css was killed but not physically removed yet followed by the
removal of the cgroup that is hosting the css. This patch prevents any
harmm from being done when that happens.

Signed-off-by: Waiman Long <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>
Cc: [e-mail] # v4.5+

include/linux/cgroup-defs.h

···
 	CSS_ONLINE	= (1 << 1), /* between ->css_online() and ->css_offline() */
 	CSS_RELEASED	= (1 << 2), /* refcnt reached zero, released */
 	CSS_VISIBLE	= (1 << 3), /* css is visible to userland */
+	CSS_DYING	= (1 << 4), /* css is dying */
 };
 
 /* bits in struct cgroup flags field */

kernel/cgroup/cgroup.c

···
 {
 	lockdep_assert_held(&cgroup_mutex);
 
+	if (css->flags & CSS_DYING)
+		return;
+
+	css->flags |= CSS_DYING;
+
 	/*
 	 * This must happen before css is disassociated with its cgroup.
 	 * See seq_css() for details.