linux-kernel - Re: [PATCH 2/2] printk: wake up klogd in vprintk

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Date:   Wed, 18 Apr 2018 10:29:43 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] printk: wake up klogd in vprintk_emit

On Sat, 14 Apr 2018 12:01:45 +0900
Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1888,6 +1888,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>  
>  	printed_len = log_output(facility, level, lflags, dict, dictlen, text, text_len);
>  
> +	wake_up_klogd();
>  	logbuf_unlock_irqrestore(flags);

You can't do this, because the scheduler can call printk_deferred()
with the rq lock held, and printk_deferred() will grab the logbuf lock.

Calling wake_up_klogd() will grab the rq lock and give us a A-B<->B-A
locking order.

-- Steve