cgroup: prevent spurious transition into non-frozen state (cb2c4cd8) · Commits · 戴 / test

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>

kernel/cgroup/freezer.c

+5 −11

Original line number	Diff line number	Diff line
		@@ -139,19 +139,13 @@ void cgroup_leave_frozen(bool always_leave)
		cgroup_update_frozen(cgrp);
		WARN_ON_ONCE(!current->frozen);
		current->frozen = false;
		} else if (!(current->jobctl & JOBCTL_TRAP_FREEZE)) {
		spin_lock(&current->sighand->siglock);
		current->jobctl \|= JOBCTL_TRAP_FREEZE;
		set_thread_flag(TIF_SIGPENDING);
		spin_unlock(&current->sighand->siglock);
		}
		spin_unlock_irq(&css_set_lock);

		if (unlikely(current->frozen)) {
		/*
		* If the task remained in the frozen state,
		* make sure it won't reach userspace without
		* entering the signal handling loop.
		*/
		spin_lock_irq(&current->sighand->siglock);
		recalc_sigpending();
		spin_unlock_irq(&current->sighand->siglock);
		}
		}

		/*

kernel/signal.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -2514,7 +2514,7 @@ relock:
		*/
		if (unlikely(cgroup_task_frozen(current))) {
		spin_unlock_irq(&sighand->siglock);
		cgroup_leave_frozen(true);
		cgroup_leave_frozen(false);
		goto relock;
		}

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