cgroup: fix CSS_TASK_ITER_PROCS
authorTejun Heo <tj@kernel.org>
Thu, 8 Nov 2018 20:15:15 +0000 (12:15 -0800)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 9 Jan 2019 16:38:45 +0000 (17:38 +0100)
commit8a2fbdd5b0204c823ec641ab9ddca23eb6fd0c33
tree3b9f06b76c92f2ff51a3ad6e454e0d916df23ff3
parent99dcd45f274e10b0a4b04f2836218762ed2294d4
cgroup: fix CSS_TASK_ITER_PROCS

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@kernel.org>
Reported-by: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Fixes: 8cfd8147df67 ("cgroup: implement cgroup v2 thread support")
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
kernel/cgroup/cgroup.c