linux-kernel - Re: [PATCH v4] panic: add an option to replay all the printk message in buffer

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Date:   Sat, 27 Apr 2019 01:43:02 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Aaro Koskinen <aaro.koskinen@...ia.com>,
        Kees Cook <keescook@...omium.org>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH v4] panic: add an option to replay all the printk message
 in buffer

On (04/26/19 16:14), Petr Mladek wrote:
>
> Then I wonder why, for example, native_stop_other_cpus() waits
> 10ms at maximum after sending the NMIs. What is the state
> of the CPUs that miss this deadline?

Well, I saw a case when CPU was forcibly powered off (embedded),
for instance.

> > - But, more importantly, if that CPUB is in atomic context, then panic
> >   CPUA will spin, waiting for that CPUB to handoff printing, before
> >   panic CPU will even try to stop all CPUs.
> > 
> > 	pr_emerg("Kernel panic - not syncing: %s\n", buf)
> > 
> >   is the point of 'synchronization' - panic CPU will wait for
> >   current console owner.
> 
> "Synchronization point" is too strong formulation.

But it is sort of synchronized. That's why console_owner patch
set solved the panic-printk deadlock which Google folks reported
a while ago.

> The console waiter logic is effective but it does not always
> work. The current console owner must be calling the console
> drivers.
>
> >   Hmm, we might have a bit of a problem here, maybe.
>
> Hmm, the printk() might wait forever when NMI stopped
> the current console owner in the console driver code
> or with the logbuf_lock taken.

I guess this is why we re-init logbuf lock from panic,
however, we don't do anything with the console_owner.

> The console waiter logic might get solved by clearing
> the console_owner in console_flush_on_panic(). It can't
> be much worse, we already ignore console_lock() there, ...

Right.

[..]
> Anyway, do we really need to have length discussion about
> whether the locks are needed? They will not break anything.

I'm not objecting v5 nor your request to add that locking there.
I'm talking about different things.

	-ss