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	Original line	Diff line number	Diff line
	@@ -139,19 +139,13 @@ void cgroup_leave_frozen(bool always_leave)
	cgroup_update_frozen(cgrp);		cgroup_update_frozen(cgrp);
	WARN_ON_ONCE(!current->frozen);		WARN_ON_ONCE(!current->frozen);
	current->frozen = false;		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);		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	Original line	Diff line number	Diff line
	@@ -2514,7 +2514,7 @@ relock:
	*/		*/
	if (unlikely(cgroup_task_frozen(current))) {		if (unlikely(cgroup_task_frozen(current))) {
	spin_unlock_irq(&sighand->siglock);		spin_unlock_irq(&sighand->siglock);
	cgroup_leave_frozen(true);		cgroup_leave_frozen(false);
	goto relock;		goto relock;
	}		}

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