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SubjectRe: [PATCH v2] exit: Move preemption fixup up, move blocking operations down
On Thu, 5 Mar 2020 23:30:13 +0100
Jann Horn <jannh@google.com> wrote:

> On Thu, Mar 5, 2020 at 11:13 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Thu, 5 Mar 2020 23:06:57 +0100
> > Jann Horn <jannh@google.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);

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