4.20-stable review patch. If anyone has any objections, please let me know.
------------------
From: Tejun Heo <tj@xxxxxxxxxx>
commit e9d81a1bc2c48ea9782e3e8b53875f419766ef47 upstream.
CSS_TASK_ITER_PROCS implements process-only iteration by making
css_task_iter_advance() skip tasks which aren't threadgroup leaders;
however, when an iteration is started css_task_iter_start() calls the
inner helper function css_task_iter_advance_css_set() instead of
css_task_iter_advance(). As the helper doesn't have the skip logic,
when the first task to visit is a non-leader thread, it doesn't get
skipped correctly as shown in the following example.
# ps -L 2030
PID LWP TTY STAT TIME COMMAND
2030 2030 pts/0 Sl+ 0:00 ./test-thread
2030 2031 pts/0 Sl+ 0:00 ./test-thread
# mkdir -p /sys/fs/cgroup/x/a/b
# echo threaded > /sys/fs/cgroup/x/a/cgroup.type
# echo threaded > /sys/fs/cgroup/x/a/b/cgroup.type
# echo 2030 > /sys/fs/cgroup/x/a/cgroup.procs
# cat /sys/fs/cgroup/x/a/cgroup.threads
2030
2031
# cat /sys/fs/cgroup/x/cgroup.procs
2030
# echo 2030 > /sys/fs/cgroup/x/a/b/cgroup.threads
# cat /sys/fs/cgroup/x/cgroup.procs
2031
2030
The last read of cgroup.procs is incorrectly showing non-leader 2031
in cgroup.procs output.
This can be fixed by updating css_task_iter_advance() to handle the
first advance and css_task_iters_tart() to call
css_task_iter_advance() instead of the inner helper. After the fix,
the same commands result in the following (correct) result:
# ps -L 2062
PID LWP TTY STAT TIME COMMAND
2062 2062 pts/0 Sl+ 0:00 ./test-thread
2062 2063 pts/0 Sl+ 0:00 ./test-thread
# mkdir -p /sys/fs/cgroup/x/a/b
# echo threaded > /sys/fs/cgroup/x/a/cgroup.type
# echo threaded > /sys/fs/cgroup/x/a/b/cgroup.type
# echo 2062 > /sys/fs/cgroup/x/a/cgroup.procs
# cat /sys/fs/cgroup/x/a/cgroup.threads
2062
2063
# cat /sys/fs/cgroup/x/cgroup.procs
2062
# echo 2062 > /sys/fs/cgroup/x/a/b/cgroup.threads
# cat /sys/fs/cgroup/x/cgroup.procs
2062
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx>
Fixes: 8cfd8147df67 ("cgroup: implement cgroup v2 thread support")
Cc: stable@xxxxxxxxxxxxxxx # v4.14+
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/cgroup/cgroup.c | 33 +++++++++++++++++++--------------
1 file changed, 19 insertions(+), 14 deletions(-)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4202,20 +4202,25 @@ static void css_task_iter_advance(struct
lockdep_assert_held(&css_set_lock);
repeat:
- /*
- * Advance iterator to find next entry. cset->tasks is consumed
- * first and then ->mg_tasks. After ->mg_tasks, we move onto the
- * next cset.
- */
- next = it->task_pos->next;
-
- if (next == it->tasks_head)
- next = it->mg_tasks_head->next;
-
- if (next == it->mg_tasks_head)
+ if (it->task_pos) {
+ /*
+ * Advance iterator to find next entry. cset->tasks is
+ * consumed first and then ->mg_tasks. After ->mg_tasks,
+ * we move onto the next cset.
+ */
+ next = it->task_pos->next;
+
+ if (next == it->tasks_head)
+ next = it->mg_tasks_head->next;
+
+ if (next == it->mg_tasks_head)
+ css_task_iter_advance_css_set(it);
+ else
+ it->task_pos = next;
+ } else {
+ /* called from start, proceed to the first cset */
css_task_iter_advance_css_set(it);
- else
- it->task_pos = next;
+ }
/* if PROCS, skip over tasks which aren't group leaders */
if ((it->flags & CSS_TASK_ITER_PROCS) && it->task_pos &&
@@ -4255,7 +4260,7 @@ void css_task_iter_start(struct cgroup_s
it->cset_head = it->cset_pos;
- css_task_iter_advance_css_set(it);
+ css_task_iter_advance(it);
spin_unlock_irq(&css_set_lock);
}
