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

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Date:   Sat, 13 Jul 2019 22:19:47 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Konstantin Khlebnikov <koct9i@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Petr Mladek <pmladek@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg_dump
 from NMI context

On (07/13/19 09:46), Konstantin Khlebnikov wrote:
> > On (07/12/19 17:54), Konstantin Khlebnikov wrote:
> 
> Yep printk() can deal with NMI, but kmsg_dump() is a different beast.
> It reads printk buffer and saves content into persistent storage like ACPI ERST.

Ah, sorry! I misread your patch. Yeah, I see what you are doing.

OK. So, I guess that for kmsg_dump(KMSG_DUMP_PANIC) we should be
fine in general.

We call kmsg_dump(KMSG_DUMP_PANIC) after smp_send_stop() and after
printk_safe_flush_on_panic(). printk_safe_flush_on_panic() resets
the state of logbuf_lock, so logbuf_lock, in general case, should
be unlocked by the time we call kmsg_dump(KMSG_DUMP_PANIC).
Even for nested contexts.

	CPU0
	printk()
	 logbuf_lock_irqsave(flags)
	  -> NMI
	   panic()
	    smp_send_stop()
	     printk_safe_flush_on_panic()
	      raw_spin_lock_init(&logbuf_lock) << reinit >>
	    kmsg_dump(KMSG_DUMP_PANIC)
	     logbuf_lock_irqsave(flags)        << expected to be OK >>

So do we have strong reasons to disable NMI->panic->kmsg_dump(DUMP_PANIC)?

Other kmsg_dump(), maybe, can experience some troubles sometimes,
need to check that.

	-ss