linux-kernel - Re: [PATCH] lib/spinlock_debug.c: prevent deadlock in spin_dump() by console

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Date:	Fri, 15 Jan 2016 10:30:50 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	akinobu.mita@...il.com, jack@...e.cz,
	torvalds@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] lib/spinlock_debug.c: prevent deadlock in spin_dump() by
 console_sem


* Byungchul Park <byungchul.park@....com> wrote:

> Deadlock occured when using CONFIG_DEBUG_SPINLOCK, in spin_dump().
> Backtrace prints printk() -> console_trylock() -> do_raw_spin_lock() ->
> spin_dump() -> printk()... infinitely.
> 
> If spin_bug() is called from a function like printk() which is using
> console lock, we should prevent the debug spinlock code from calling
> printk() any more in the context.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/locking/spinlock_debug.c | 10 ++++++++++
>  kernel/printk/printk.c          |  5 +++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index 0374a59..b339c5e 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -67,11 +67,21 @@ static void spin_dump(raw_spinlock_t *lock, const char *msg)
>  	dump_stack();
>  }
>  
> +extern int is_console_lock(raw_spinlock_t *lock);
> +
>  static void spin_bug(raw_spinlock_t *lock, const char *msg)
>  {
>  	if (!debug_locks_off())
>  		return;
>  
> +	/*
> +	 * If this function is called from a function like printk()
> +	 * which is trying console lock, we cannot call printk() any
> +	 * more. Or it's obviously deadlock!

Small spelling nit, the last sentence is not correct, pick one of these variants:

  'It's an obvious deadlock.'
  'It's obviously a deadlock.'

Thanks,

	Ingo