linux-kernel - Re: [PATCH 2/2] printk/panic/x86: Allow to access printk log buffer after crash_smp_send

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Message-ID: <CALYGNiMnqUKxKsY1JRi075xs-P_QzfA4Pg3XANiW0mFYkp_RQQ@mail.gmail.com>
Date:   Thu, 18 Jul 2019 14:07:20 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Ogness <john.ogness@...utronix.de>,
        Petr Tesarik <ptesarik@...e.cz>, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] printk/panic/x86: Allow to access printk log buffer
 after crash_smp_send_stop()

On Thu, Jul 18, 2019 at 1:48 PM Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> 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