linux-kernel - Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg

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Message-ID: <20190713060929.GB1038@tigerII.localdomain>
Date:   Sat, 13 Jul 2019 15:09:29 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg_dump
 from NMI context

On (07/12/19 17:54), Konstantin Khlebnikov wrote:
> Function kmsg_dump could be invoked from NMI context intentionally or
> accidentally because it is called at various oops/panic paths.
> Kernel message dumpers are not ready to work in NMI context right now.
> They could deadlock on lockbuf_lock or break internal structures.

Hmm.
printk()-s from NMI go through per-CPU printk_safe/nmi - a bunch of
lockless buffers which is supposed to deal with printk() deadlocks,
including NMI printk()-s.

include/linux/hardirq.h

#define nmi_enter()
	...
	printk_nmi_enter();
	...

#define nmi_exit()
	...
	printk_nmi_exit();
	...

So we are not really supposed to deadlock.

	-ss