linux-kernel - Re: [RFC][PATCH 0/2] Access console drivers list under console

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Message-ID: <20190424054305.GA28894@jagdpanzerIV>
Date:   Wed, 24 Apr 2019 14:43:05 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 0/2] Access console drivers list under console_sem

On (04/23/19 09:48), Steven Rostedt wrote:
> > 	RFC
> > 
> > 	Normally, we grab console_sem lock before we iterate consoles
> > list, which is necessary if we want to be race free. The only exception
> > to this rule is console_flush_on_panic(). However, it seems that we are
> > not fully race free - register_console() iterates console drivers list
> > in unsafe manner in several places. E.g. the following scenarion:
> > 
> > 	CPU0					CPU1
> > 	register_console()			unregister_console()
> > 						 console_lock()
> > 	  for_each_console()			  // modify console_drivers
> > 	    con->foo				    kfree(con)
> > 
> > So I have two quick-n-dirty patches, which remove unsafe console list
> > access.
> > 
> > What do you think?
> 
> I just skimmed the patches and haven't done a thorough review, but the
> concept seems sane to me.

Thank you Steven. Let me know if anything doesn't work for you.

I have vague memories of a kernel Oops at con->foo dereferencing
(saw a message on linux-rt-users list a while ago, if I'm not
mistaken). And it seems that we have some races in printk code.

	-ss