linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Date:   Wed, 13 Mar 2019 11:15:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sebastian Siewior <bigeasy@...utronix.de>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On (03/12/19 16:15), John Ogness wrote:
> > I suggest the following way forward (separate patchsets):
> >
> >     1. Replace log buffer (least controversial thing)
> 
> Yes. I will post a series that only implements the ringbuffer using your
> simplified API. That will be enough to remove printk_safe and actually
> does most of the work of updating devkmsg, kmsg_dump, and syslog.

This may _not_ be enough to remove printk_safe. One of the reasons
printk_safe "condom" came into existence was console_sem (which
is a bit too important to ignore it):

	printk()
	 console_trylock()
	  console_unlock()
	   up()
	    raw_spin_lock_irqsave(&sem->lock, flags)
	     __up()
	      wake_up_process()
	       WARN/etc
	        printk()
		 console_trylock()
		  down_trylock()
		   raw_spin_lock_irqsave(&sem->lock, flags)  << deadlock

Back then we were looking at

	printk->console_sem->lock->printk->console_sem->lock

deadlock report from LG, if I'm not mistaken.

	-ss