• Oleg Nesterov's avatar
    freezer: change ptrace_stop/do_signal_stop to use freezable_schedule() · 5d8f72b5
    Oleg Nesterov authored
    try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks
    to ensure that a task doing STOPPED/TRACED -> RUNNING transition
    can't escape freezing. This mostly works, but ptrace_stop() does
    not necessarily call schedule(), it can change task->state back to
    RUNNING and check freezing() without any lock/barrier in between.
    
    We could add the necessary barrier, but this patch changes
    ptrace_stop() and do_signal_stop() to use freezable_schedule().
    This fixes the race, freezer_count() and freezer_should_skip()
    carefully avoid the race.
    
    And this simplifies the code, try_to_freeze_tasks/update_if_frozen
    no longer need to use task_is_stopped_or_traced() checks with the
    non trivial assumptions. We can rely on the mechanism which was
    specially designed to mark the sleeping task as "frozen enough".
    
    v2: As Tejun pointed out, we can also change get_signal_to_deliver()
    and move try_to_freeze() up before 'relock' label.
    Signed-off-by: 's avatarOleg Nesterov <oleg@redhat.com>
    Signed-off-by: 's avatarTejun Heo <tj@kernel.org>
    5d8f72b5