Re: [PATCH] exit: Move preemption fixup up, move blocking operations down

[Jann Horn]

On Thu, Mar 5, 2020 at 11:00 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> With CONFIG_DEBUG_ATOMIC_SLEEP=y and CONFIG_CGROUPS=y, kernel oopses in
> non-preemptible context look untidy; after the main oops, the kernel prints
> a "sleeping function called from invalid context" report because
> exit_signals() -> cgroup_threadgroup_change_begin() -> percpu_down_read()
> can sleep, and that happens before the preempt_count_set(PREEMPT_ENABLED)
> fixup.
>
> It looks like the same thing applies to profile_task_exit() and
> kcov_task_exit().
>
> Fix it by moving the preemption fixup up and the calls to
> profile_task_exit() and kcov_task_exit() down.
>
> Fixes: 1dc0fffc48af ("sched/core: Robustify preemption leak checks")
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> As so often, I have no idea which tree this should go through. tip? mm?
>
>  kernel/exit.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 2833ffb0c211..db77c540aa92 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -713,8 +713,12 @@ void __noreturn do_exit(long code)
>         struct task_struct *tsk = current;
>         int group_dead;
>
> -       profile_task_exit(tsk);
> -       kcov_task_exit(tsk);
> +       /*
> +        * We can get here from a kernel oops, sometimes with preemption off.
> +        * Start by checking for critical errors.
> +        * Then fix up important state like USER_DS and preemption.
> +        * Then do everything else.
> +        */
>
>         WARN_ON(blk_needs_flush_plug(tsk));
>
> @@ -732,6 +736,17 @@ void __noreturn do_exit(long code)
>          */
>         set_fs(USER_DS);
>
> +       if (unlikely(in_atomic())) {
> +               pr_info("note: %s[%d] exited with preempt_count %d\n",
> +                       current->comm, task_pid_nr(current),
> +                       preempt_count());
> +               preempt_count_set(PREEMPT_ENABLED);
> +       }
> +
> +       profile_task_exit(tsk);
> +       kcov_task_exit(tsk);
> +
> +
>         ptrace_event(PTRACE_EVENT_EXIT, code);

Ugh, I don't know where that extra whitespace comes from... I'll resend.