Commit e9d81a1b authored Nov 08, 2018 by Tejun Heo

cgroup: fix CSS_TASK_ITER_PROCS



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@kernel.org>
Reported-by: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Fixes: 8cfd8147 ("cgroup: implement cgroup v2 thread support")
Cc: stable@vger.kernel.org # v4.14+

parent f2ce1065

kernel/cgroup/cgroup.c

+17 −12

Original line number	Diff line number	Diff line
		@@ -4202,10 +4202,11 @@ static void css_task_iter_advance(struct css_task_iter *it)

		lockdep_assert_held(&css_set_lock);
		repeat:
		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.
		* 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;

		@@ -4216,6 +4217,10 @@ repeat:
		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);
		}

		/* 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_subsys_state *css, unsigned int flags,

		it->cset_head = it->cset_pos;

		css_task_iter_advance_css_set(it);
		css_task_iter_advance(it);

		spin_unlock_irq(&css_set_lock);
		}

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