linux-kernel - Re: [PATCH v5] lib/spinlock_debug.c: prevent a recursive cycle in the debug code

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Date:	Mon, 1 Feb 2016 11:31:12 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Byungchul Park <byungchul.park@....com>
Cc:	akpm@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	sergey.senozhatsky.work@...il.com, peter@...leysoftware.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v5] lib/spinlock_debug.c: prevent a recursive cycle in
 the debug code

On (01/29/16 21:43), Byungchul Park wrote:
[..]
> +extern int is_console_lock(raw_spinlock_t *lock);
> +
>  static void __spin_lock_debug(raw_spinlock_t *lock)
>  {
>  	u64 i;
> @@ -113,11 +115,19 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
>  			return;
>  		__delay(1);
>  	}
> -	/* lockup suspected: */
> -	spin_dump(lock, "lockup suspected");
> +
> +	/*
> +	 * If this function is called from printk(), then we should
> +	 * not call printk() more. Or it will cause an infinite
> +	 * recursive cycle!
> +	 */
> +	if (likely(!is_console_lock(lock))) {
> +		/* lockup suspected: */
> +		spin_dump(lock, "lockup suspected");
>  #ifdef CONFIG_SMP
> -	trigger_all_cpu_backtrace();
> +		trigger_all_cpu_backtrace();
>  #endif
> +	}

/* speaking in a context of printk locks only */

... may be for a recoverable "lockup suspected" spin_dump()
we can switch to deferred printk of the messages, which is a
bit better than nothing. but for unrecoverable "lockup suspected"
spin_dump() -- an actual bug (spin lock owner is not going to
release the lock any more) -- we need something else, I think.
the bug will neither be reported nor fixed.

	-ss

>  	/*
>  	 * The trylock above was causing a livelock.  Give the lower level arch
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2ce8826..568ab11 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1981,6 +1981,11 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
>  }
>  #endif
>  
> +int is_console_lock(raw_spinlock_t *lock)
> +{
> +	return lock == &console_sem.lock;
> +}
> +
>  static int __add_preferred_console(char *name, int idx, char *options,
>  				   char *brl_options)
>  {
> -- 
> 1.9.1
>