linux-kernel - Re: [PATCH v2] exit: Move preemption fixup up, move blocking operations down

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Date:   Thu, 5 Mar 2020 17:44:23 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] exit: Move preemption fixup up, move blocking
 operations down

On Thu, 5 Mar 2020 23:30:13 +0100
Jann Horn <jannh@...gle.com> wrote:

> On Thu, Mar 5, 2020 at 11:13 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Thu,  5 Mar 2020 23:06:57 +0100
> > Jann Horn <jannh@...gle.com> 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.  
> [...]
> > > +     if (unlikely(in_atomic())) {
> > > +             pr_info("note: %s[%d] exited with preempt_count %d\n",
> > > +                     current->comm, task_pid_nr(current),
> > > +                     preempt_count());  
> >
> > This should be more than a pr_info. It should also probably state the
> > "Dazed and confused, best to reboot" message.
> >
> > Because if something crashed in a non preempt section, it may likely be
> > holding a lock that it will never release, causing a soon to be deadlock!  
> 
> I didn't write that code, I'm just moving it around. :P But I guess if

Ah, I didn't scroll down enough to see it was just moved.

> you want, I can change it in the same patch... something like this on
> top? Does that look reasonable?

No, an update to the text should be done as a separate patch, as it is a
different type of change.

Thanks,

-- Steve

> 
>         if (unlikely(in_atomic())) {
> -               pr_info("note: %s[%d] exited with preempt_count %d\n",
> +               pr_emerg("note: %s[%d] exited with preempt_count %d,
> system might deadlock, please reboot\n",
>                         current->comm, task_pid_nr(current),
>                         preempt_count());
>                 preempt_count_set(PREEMPT_ENABLED);