linux-kernel - Re: [PATCH 2/4] printk: disable optimistic spin during panic

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Date:   Wed, 26 Jan 2022 19:06:44 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Stephen Brennan <stephen.s.brennan@...cle.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] printk: disable optimistic spin during panic

On (22/01/26 10:51), John Ogness wrote:
> > Is there something that prevents panic CPU from NMI hlt CPU which is
> > in console_trylock() under raw_spin_lock_irqsave()?
> >
> >  CPU0				CPU1
> > 				console_trylock_spinnning()
> > 				 console_trylock()
> > 				  down_trylock()
> > 				   raw_spin_lock_irqsave(&sem->lock)
> >
> >  panic()
> >   crash_smp_send_stop()
> >    NMI 			-> 		HALT
> 
> This is a good point. I wonder if console_flush_on_panic() should
> perform a sema_init() before it does console_trylock().

A long time ago there was zap_locks() function in printk, that used
to re-init console semaphore and logbuf spin_lock, but _only_ in case
of printk recursion (which was never reliable)

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/printk/printk.c?h=v4.9.297#n1557

This has been superseded by printk_safe per-CPU buffers so we removed
that function.

So it could be that may be we want to introduce something similar to
zap_locks() again.

All reasonable serial consoles drivers should take oops_in_progress into
consideration in ->write(), so we probably don't care for console_drivers
spinlocks, etc. but potentially can do a bit better on the printk side.