Re: [PATCH 2/2] printk/panic/x86: Allow to access printk log buffer after crash_smp_send_stop()

[Konstantin Khlebnikov]

On Thu, Jul 18, 2019 at 1:48 PM Sergey Senozhatsky
<sergey.senozhatsky.work@xxxxxxxxx> wrote:
>
> On (07/16/19 09:28), Petr Mladek wrote:
> [..]
> > +int printk_bust_lock_safe(bool kdump_smp_stop)
> >  {
> >       if (!raw_spin_is_locked(&logbuf_lock))
> >               return 0;
> >
> > -     if (num_online_cpus() == 1) {
> > +     if (num_online_cpus() == 1 || kdump_smp_stop)  {
> >               debug_locks_off();
> >               raw_spin_lock_init(&logbuf_lock);
> >               return 0;
>
> Let me test the waters. Criticize the following idea:
>
> Can we, sort of, disconnect "supposed to be dead" CPUs from printk()
> so then we can unconditionally re-init printk() from panic-CPU?
>
> We have per-CPU printk_state; so panic-CPU can set, let's say,
> DEAD_CPUS_TELL_NO_TALES bit on all CPUs but self, and vprintk_func()
> will do nothing if DEAD_CPUS_TELL_NO_TALES bit set on particular
> CPU. Foreign CPUs are not even supposed to be alive, and smp_send_stop()
> waits for IPI acks from secondary CPUs long enough on average (need
> to check that) so if one of the CPUs is misbehaving and doesn't want
> to die (geez...) we will just "disconnect" it from printk() to minimize
> possible logbuf/console drivers interventions and then proceed with
> panic; assuming that misbehaving CPUs are actually up to something
> sane. Sometimes, you know, in some cases, those CPUs are already dead:
> either accidentally powered off, or went completely nuts and do nothing,
> etc. etc. but we still can kdump() and console_flush_on_panic().

Good idea.
Panic-CPU could just increment state to reroute printk into 'safe'
per-cpu buffer.

>
>         -ss