linux-kernel - Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the debug code

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Message-ID: <20160128071502.GA31266@X58A-UD3R>
Date:	Thu, 28 Jan 2016 16:15:02 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	akpm@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in
 the debug code

On Wed, Jan 27, 2016 at 02:49:35PM -0800, Peter Hurley wrote:
> And we already have lockdep turned off to avoid triggering a recursive
> lockdep report (which I think is a mistake).

Yes, we already have a way to turn off the lock debug so that we can
avoid it. So I used it in v4.

thanks,
byungchul

> 
> I think we should be working toward properly handling recursion
> in printk().
> 
> Regards,
> Peter Hurley
> 
> 
> > When the debug spinlock code is called from printk(), we should prevent
> > calling spin_dump() and the like, calling printk() again in that context.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > ---
> >  include/linux/debug_locks.h     |  4 ++++
> >  kernel/locking/spinlock_debug.c | 14 +++++++++++---
> >  2 files changed, 15 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/debug_locks.h b/include/linux/debug_locks.h
> > index 822c135..b0f977e 100644
> > --- a/include/linux/debug_locks.h
> > +++ b/include/linux/debug_locks.h
> > @@ -10,6 +10,10 @@ struct task_struct;
> >  extern int debug_locks;
> >  extern int debug_locks_silent;
> >  
> > +static inline void __debug_locks_on(void)
> > +{
> > +	debug_locks = 1;
> > +}
> >  
> >  static inline int __debug_locks_off(void)
> >  {
> > diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> > index 0374a59..65177ba 100644
> > --- a/kernel/locking/spinlock_debug.c
> > +++ b/kernel/locking/spinlock_debug.c
> > @@ -113,11 +113,19 @@ static void __spin_lock_debug(raw_spinlock_t *lock)
> >  			return;
> >  		__delay(1);
> >  	}
> > -	/* lockup suspected: */
> > -	spin_dump(lock, "lockup suspected");
> > +
> > +	/*
> > +	 * We should prevent calling printk() further, since it would cause
> > +	 * an infinite recursive cycle if it's called from printk()!
> > +	 */
> > +	if (__debug_locks_off()) {
> > +		/* lockup suspected: */
> > +		spin_dump(lock, "lockup suspected");
> >  #ifdef CONFIG_SMP
> > -	trigger_all_cpu_backtrace();
> > +		trigger_all_cpu_backtrace();
> >  #endif
> > +		__debug_locks_on();
> > +	}
> >  
> >  	/*
> >  	 * The trylock above was causing a livelock.  Give the lower level arch
> >