linux-kernel - Re: [PATCH v2] lib/spinlock_debug.c: prevent an infinite recursive cycle in spin

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Date:	Tue, 26 Jan 2016 16:11:24 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	akinobu.mita@...il.com, jack@...e.cz, torvalds@...ux-foundation.org
Subject: Re: [PATCH v2] lib/spinlock_debug.c: prevent an infinite recursive
 cycle in spin_dump()

On Mon, 18 Jan 2016 09:58:12 +0900 Byungchul Park <byungchul.park@....com> wrote:

> It causes an infinite recursive cycle when using CONFIG_DEBUG_SPINLOCK,
> in the spin_dump(). Backtrace prints printk() -> console_trylock() ->
> do_raw_spin_lock() -> spint_bug() -> spin_dump() -> printk()...
> infinitely.
> 
> If the spin_bug() is called from a function like printk() which is
> trying to obtain the console lock, we should prevent the debug spinlock
> code from calling printk() again in that context.
> 

lol.  Excellent.

> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -67,11 +67,22 @@ 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 to obtain the console lock, then we should
> +	 * not call printk() any more. Or it will cause an infinite
> +	 * recursive cycle!
> +	 */
> +	if (unlikely(is_console_lock(lock)))
> +		return;
> +
>  	spin_dump(lock, msg);
>  }
>  
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2ce8826..50ea552 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -119,6 +119,11 @@ static int __down_trylock_console_sem(unsigned long ip)
>  	up(&console_sem);\
>  } while (0)
>  
> +int is_console_lock(raw_spinlock_t *lock)
> +{
> +	return &console_sem.lock == lock;
> +}
> +
>  /*
>   * This is used for debugging the mess that is the VT code by
>   * keeping track if we have the console semaphore held. It's

I can't immediately think of anything better than this.  It's a hack, but
it's a small and quite clear hack.