[Devel] [PATCH rh7] kernel/freezer: don't freeze stopped & about to be ptraced task

[Andrey Ryabinin]

So, the ptrace() hangs if we try to attach to stopped task
from freezing cgroup:

Tracee:                                               Tracer:
static bool do_signal_stop(int signr)
    __set_current_state(TASK_STOPPED);
        freezable_schedule();
             freezer_do_not_count();
             schedule(); /* waiting for wake up */

                                                      ptrace_attach()
                                                          if (task_is_stopped(task) &&
                                                                 task_set_jobctl_pending(task,
								     JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
                                                              signal_wake_up_state(task, __TASK_STOPPED);
             /* woken up by ptrace_attach() */
             freezer_count();
                __refrigerator()
                                                          /* And here we will hang, because tracee
							   *  is now frozen in __refrigerator() */
                                                           wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT,
                                                                       TASK_UNINTERRUPTIBLE);

The right fix for this would be to rework cgroup freezer,
but for now, we could workaround this by forbiding to freeze task
if JOBCTL_TRAPPING_BIT is set.
ptrace_attach() is the only place which sets that bit iff
task_is_stopped(), thus TRAPPING_BIT is clear indication that ptrace_attach
waits for clearing that bit => we shouldn't freeze.

https://jira.sw.ru/browse/PSBM-40683

Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 kernel/freezer.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/freezer.c b/kernel/freezer.c
index f6da038..3445d5d 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -39,6 +39,9 @@ bool freezing_slow_path(struct task_struct *p)
 	if (test_thread_flag(TIF_MEMDIE))
 		return false;
 
+	if (p->jobctl & JOBCTL_TRAPPING_BIT)
+		return false;
+
 	if (pm_nosig_freezing || cgroup_freezing(p))
 		return true;
 
-- 
2.4.10