linux-kernel - Re: [PATCH v5 1/4] printk/nmi: generic solution for safe printk in NMI

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Date:   Wed, 19 Apr 2017 19:21:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <rmk+kernel@....linux.org.uk>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jiri Kosina <jkosina@...e.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Chris Metcalf <cmetcalf@...hip.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        adi-buildroot-devel@...ts.sourceforge.net,
        linux-cris-kernel@...s.com, linux-mips@...ux-mips.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        Jan Kara <jack@...e.cz>, Ralf Baechle <ralf@...ux-mips.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH v5 1/4] printk/nmi: generic solution for safe printk in
 NMI

On Wed, Apr 19, 2017 at 01:13:41PM -0400, Steven Rostedt wrote:
> On Thu, 21 Apr 2016 13:48:42 +0200
> Petr Mladek <pmladek@...e.com> wrote:
> 
> > printk() takes some locks and could not be used a safe way in NMI context.
> 
> I just found a problem with this solution. It kills ftrace dumps from
> NMI context :-(
> 
> [ 1295.168495]    <...>-67423  10dNh1 382171111us : do_raw_spin_lock <-_raw_spin_lock
> [ 1295.168495]    <...>-67423  10dNh1 382171111us : sched_stat_runtime: comm=cc1 pid=67423 runtime=96858 [ns] vruntime=11924198270 [ns]
> [ 1295.168496]    <...>-67423  10dNh1 382171111us : lock_acquire: ffffffff81c5c940 read rcu_read_lock
> [ 1295.168497]
> [ 1295.168498] Lost 4890096 message(s)!
> [ 1296.805063] ---[ end Kernel panic - not syncing: Hard LOCKUP
> [ 1296.811553] unchecked MSR access error: WRMSR to 0x83f (tried to write 0x00000000000000f6) at rIP: 0xffffffff81046fc7 (native_apic_msr_write+0x27/0x40)
> [ 1296.811553] Call Trace:
> [ 1296.811553]  <NMI>
> 
> I was hoping to see a cause of a hard lockup by enabling
> ftrace_dump_on_oops. But as NMIs now have a very small buffer that
> gets flushed, we need to find a new way to print out the full ftrace
> buffer over serial.
> 
> Thoughts?

early_printk ;-)