linux-kernel - Re: [PATCH] Fix lockup on panic with lockdep

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Date:	Tue, 13 May 2014 13:29:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Derek Basehore <dbasehore@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Joe Perches <joe@...ches.com>,
	Petr Mladek <pmladek@...e.cz>,
	Arun KS <arunks.linux@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Sonny Rao <sonnyrao@...omium.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] Fix lockup on panic with lockdep

On Mon, 12 May 2014 17:41:54 -0700 Derek Basehore <dbasehore@...omium.org> wrote:

> If we don't call mutex_acquire at the beginning of console_unblank, we can run
> into a lockup on the logbuf_lock between console_unlock and printk during panic.
> What happens in console_unlock is:
> 
> -locks logbuf_lock
> -calls mutex_release
>  -which calls printk
>   -which locks logbuf_lock
> 
> This fixes the problem by calling console_trylock (which calls mutex_acquire)
> instead of directly accessing the semaphore in console_unblank and moves the
> mutex_release to after we unlock logbuf_lock in console_unlock (interrupts are
> still disabled).

Please take a look at linux-next, where this code has changed a lot.

> index 7228258..c599ab5 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2084,7 +2084,6 @@ skip:
>  		local_irq_restore(flags);
>  	}
>  	console_locked = 0;
> -	mutex_release(&console_lock_dep_map, 1, _RET_IP_);
>  
>  	/* Release the exclusive_console once it is used */
>  	if (unlikely(exclusive_console))
> @@ -2092,6 +2091,7 @@ skip:
>  
>  	raw_spin_unlock(&logbuf_lock);
>  
> +	mutex_release(&console_lock_dep_map, 1, _RET_IP_);
>  	up(&console_sem);
>  
>  	/*
> @@ -2137,7 +2137,7 @@ void console_unblank(void)
>  	 * oops_in_progress is set to 1..
>  	 */
>  	if (oops_in_progress) {
> -		if (down_trylock(&console_sem) != 0)
> +		if (!console_trylock())
>  			return;
>  	} else
>  		console_lock();

Although the code is quite different, an equivalent change appears to
have been made already.  I'm not sure that bugfix was intentional ;)

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