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

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Message-ID: <20190423094801.377b8059@gandalf.local.home>
Date:   Tue, 23 Apr 2019 09:48:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     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 Tue, 23 Apr 2019 15:25:09 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:

> Petr, Steven,
> 
> 	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.

-- Steve

> 
> Sergey Senozhatsky (2):
>   printk: lock console_sem before we unregister boot consoles
>   printk: take console_sem when accessing console drivers list
> 
>  kernel/printk/printk.c | 117 ++++++++++++++++++++++++-----------------
>  1 file changed, 69 insertions(+), 48 deletions(-)
>