linux-kernel - Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.

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Date:   Wed, 7 Nov 2018 16:19:00 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep
 messages.

On Fri 2018-11-02 22:31:57, Tetsuo Handa wrote:
> syzbot is sometimes getting mixed output like below due to concurrent
> printk(). Mitigate such output by using line-buffered printk() API.
> 
> @@ -2421,18 +2458,20 @@ static void check_chain_key(struct task_struct *curr)
>  print_usage_bug_scenario(struct held_lock *lock)
>  {
>  	struct lock_class *class = hlock_class(lock);
> +	struct printk_buffer *buf = get_printk_buffer();
>  
>  	printk(" Possible unsafe locking scenario:\n\n");
>  	printk("       CPU0\n");
>  	printk("       ----\n");
> -	printk("  lock(");
> -	__print_lock_name(class);
> -	printk(KERN_CONT ");\n");
> +	printk_buffered(buf, "  lock(");
> +	__print_lock_name(class, buf);
> +	printk_buffered(buf, ");\n");
>  	printk("  <Interrupt>\n");
> -	printk("    lock(");
> -	__print_lock_name(class);
> -	printk(KERN_CONT ");\n");
> +	printk_buffered(buf, "    lock(");
> +	__print_lock_name(class, buf);
> +	printk_buffered(buf, ");\n");
>  	printk("\n *** DEADLOCK ***\n\n");
> +	put_printk_buffer(buf);
>  }
>  
>  static int

I really hope that the maze of pr_cont() calls in lockdep.c is the most
complicated one that we would meet.

Anyway, the following comes to my mind:

1. The mixing of normal and buffered printk calls is a bit confusing
   and error prone. It would make sense to use the buffered printk
   everywhere in the given section of code even when it is not
   strictly needed.

2. I would replace "buf" with "pbuf" or "prbuf" to distinguish it a
   bit from other eventual buffers.


Best Regards,
Petr