Re: [PATCH v2 2/3] freezer: do not send a fake signal to aPF_DUMPCORE thread

[Mandeep Singh Baines]

On Sun, Feb 24, 2013 at 10:36 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> A coredumping thread can't be frozen anyway but the fake signal sent
> by freeze_task() can confuse dump_write/wait_for_dump_helpers/etc
> and interrupt the coredump.
>
> We are going to make the do_coredump() paths freezable but the fake
> TIF_SIGPENDING doesn't help, it only makes sense when we assume that
> the target can return to user-mode and call get_signal_to_deliver().
>
> Change freeze_task() to check PF_DUMPCORE along with PF_KTHREAD. We
> need to recheck PF_DUMPCORE under ->siglock to avoid the race with
> zap_threads() which can set this flag right before we take the lock.
>

Won't this prevent suspend?

If there is a wait_event_interruptible in the coredump path, you'll
wake it up but it will simply go back to sleep. So try_to_freeze_tasks
will fail waiting for the coredump thread to enter the freezer.

Regards,
Mandeep

> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/freezer.c |   19 ++++++++++++-------
>  1 files changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index c38893b..595afab 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -85,14 +85,21 @@ bool __refrigerator(bool check_kthr_stop)
>  }
>  EXPORT_SYMBOL(__refrigerator);
>
> -static void fake_signal_wake_up(struct task_struct *p)
> +static bool fake_signal_wake_up(struct task_struct *p)
>  {
>         unsigned long flags;
> +       bool ret = false;
> +
> +       if (p->flags & (PF_KTHREAD | PF_DUMPCORE))
> +               return ret;
>
>         if (lock_task_sighand(p, &flags)) {
> -               signal_wake_up(p, 0);
> +               ret = !(p->flags & PF_DUMPCORE);
> +               if (ret)
> +                       signal_wake_up(p, 0);
>                 unlock_task_sighand(p, &flags);
>         }
> +       return ret;
>  }
>
>  /**
> @@ -100,8 +107,8 @@ static void fake_signal_wake_up(struct task_struct *p)
>   * @p: task to send the request to
>   *
>   * If @p is freezing, the freeze request is sent either by sending a fake
> - * signal (if it's not a kernel thread) or waking it up (if it's a kernel
> - * thread).
> + * signal (if it's not a kernel thread or a coredumping thread) or waking
> + * it up otherwise.
>   *
>   * RETURNS:
>   * %false, if @p is not freezing or already frozen; %true, otherwise
> @@ -116,9 +123,7 @@ bool freeze_task(struct task_struct *p)
>                 return false;
>         }
>
> -       if (!(p->flags & PF_KTHREAD))
> -               fake_signal_wake_up(p);
> -       else
> +       if (!fake_signal_wake_up(p))
>                 wake_up_state(p, TASK_INTERRUPTIBLE);
>
>         spin_unlock_irqrestore(&freezer_lock, flags);
> --
> 1.5.5.1
>
>
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