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

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Message-ID: <CAG48ez0UGBot8xe10pWW_bFTyFOhmQaMVpBJjEmtfM4CqdcF5w@mail.gmail.com>
Date:   Thu, 5 Mar 2020 23:30:13 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, 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
you want, I can change it in the same patch... something like this on
top? Does that look reasonable?

        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);