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

Byungchul Park
2016-01-28T07:15:29Z

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@lge.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
> >