cgroup: prevent spurious transition into non-frozen state
If freezing of a cgroup races with waking of a task from
the frozen state (like waiting in vfork() or in do_signal_stop()),
a spurious transition of the cgroup state can happen.
The task enters cgroup_leave_frozen(true), the cgroup->nr_frozen_tasks
counter decrements, and the cgroup is switched to the unfrozen state.
To prevent it, let's reserve cgroup_leave_frozen(true) for
terminating processes and use cgroup_leave_frozen(false) otherwise.
To avoid busy-looping in the signal handling loop waiting
for JOBCTL_TRAP_FREEZE set from the cgroup freezing path,
let's do it explicitly in cgroup_leave_frozen(), if the task
is going to stay frozen.
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/signal.c b/kernel/signal.c
index 095e0fc..16b72f4 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2514,7 +2514,7 @@ bool get_signal(struct ksignal *ksig)
*/
if (unlikely(cgroup_task_frozen(current))) {
spin_unlock_irq(&sighand->siglock);
- cgroup_leave_frozen(true);
+ cgroup_leave_frozen(false);
goto relock;
}