linux-kernel - Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async

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Date:	Fri, 18 Mar 2016 16:11:30 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Byungchul Park <byungchul.park@....com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Petr Mladek <pmladek@...e.com>,
	Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async

On (03/18/16 14:49), Byungchul Park wrote:
[..]
> > http://marc.info/?l=linux-kernel&m=145750373530161
> 
> I checked it now. Do you mean the wake_up_process() introduced in the new
> patch in console_unlock()? If so, I also think it does not make a deadlock,
> just can make a recursion in the worst case. I thought it was the
> wake_up_process() in up() which is eventually called from console_unlock().
> A deadlock can happen with the wake_up_proces() in up(). :-)

I'm not addressing already existing problems here. I'm trying to
minimise the impact of new code only.

[..]
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index fd24588..30559c6 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -138,14 +138,25 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
>  {
>  	u64 i;
>  	u64 loops = loops_per_jiffy * HZ;
> +	static raw_spinlock_t *suspected_lock = NULL;

this has no chances to survive on SMP systems that have spin_lockup-ed on at
least two different spin locks.

I'd really prefer not to mix-in spin_dump/printk recursion problems into this
patch set. it makes sense not to make printk recursion detection worse due to
newly added spin_locks to vprintk_emit(), but that's it. this patch set set is
fixing other things in the first place.

	-ss

>  	for (i = 0; i < loops; i++) {
>  		if (arch_spin_trylock(&lock->raw_lock))
>  			return;
>  		__delay(1);
>  	}
> -	/* lockup suspected: */
> -	spin_dump(lock, "lockup suspected");
> +
> +	/*
> +	 * When we suspect a lockup, it's good enough to inform it once for
> +	 * the same lock. Otherwise it could cause an infinite recursion if
> +	 * it's within printk().
> +	 */
> +	if (suspected_lock != lock) {
> +		suspected_lock = lock;
> +		/* lockup suspected: */
> +		spin_dump(lock, "lockup suspected");
> +		suspected_lock = NULL;
> +	}
>  #ifdef CONFIG_SMP
>  	trigger_all_cpu_backtrace();
>  #endif
> -- 
> 1.9.1
>