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

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Date:   Wed, 13 Mar 2019 09:19:32 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.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 2019-03-13, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> 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.

The main drawback of printk_safe is the safe buffers, which, aside from
bogus timestamping, may never make it back to the printk log buffer.

With the new ring buffer the safe buffers are not needed, even in the
recursive situation. As you are pointing out, the notification/wake
component of printk_safe will still be needed. I will leave that (small)
part in printk_safe.c.

John Ogness