Re: [PATCH RT 2/2 v2] list_bl: avoid BUG when the list is not locked

[Mikulas Patocka]

On Wed, 13 Nov 2019, Nikos Tsironis wrote:

> On 11/12/19 6:16 PM, Mikulas Patocka wrote:
> > list_bl would crash with BUG() if we used it without locking. dm-snapshot 
> > uses its own locking on realtime kernels (it can't use list_bl because 
> > list_bl uses raw spinlock and dm-snapshot takes other non-raw spinlocks 
> > while holding bl_lock).
> > 
> > To avoid this BUG, we must set LIST_BL_LOCKMASK = 0.
> > 
> > This patch is intended only for the realtime kernel patchset, not for the 
> > upstream kernel.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > 
> > Index: linux-rt-devel/include/linux/list_bl.h
> > ===================================================================
> > --- linux-rt-devel.orig/include/linux/list_bl.h	2019-11-07 14:01:51.000000000 +0100
> > +++ linux-rt-devel/include/linux/list_bl.h	2019-11-08 10:12:49.000000000 +0100
> > @@ -19,7 +19,7 @@
> >   * some fast and compact auxiliary data.
> >   */
> >  
> > -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
> > +#if (defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)) && !defined(CONFIG_PREEMPT_RT_BASE)
> >  #define LIST_BL_LOCKMASK	1UL
> >  #else
> >  #define LIST_BL_LOCKMASK	0UL
> > @@ -161,9 +161,6 @@ static inline void hlist_bl_lock(struct
> >  	bit_spin_lock(0, (unsigned long *)b);
> >  #else
> >  	raw_spin_lock(&b->lock);
> > -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
> > -	__set_bit(0, (unsigned long *)b);
> > -#endif
> >  #endif
> >  }
> >  
> 
> Hi Mikulas,
> 
> I think removing __set_bit()/__clear_bit() breaks hlist_bl_is_locked(),
> which is used by the RCU variant of list_bl.
> 
> Nikos

OK. so I can remove this part of the patch.

Mikulas

> > @@ -172,9 +169,6 @@ static inline void hlist_bl_unlock(struc
> >  #ifndef CONFIG_PREEMPT_RT_BASE
> >  	__bit_spin_unlock(0, (unsigned long *)b);
> >  #else
> > -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
> > -	__clear_bit(0, (unsigned long *)b);
> > -#endif
> >  	raw_spin_unlock(&b->lock);
> >  #endif
> >  }
> > 
>