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 11/13/19 1:16 PM, Mikulas Patocka wrote:
> > 
> > 
> > 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.
> > 
> 
> I think this causes another problem. LIST_BL_LOCKMASK is used in various
> functions to set/clear the lock bit, e.g. in hlist_bl_first(). So, if we
> lock the list through hlist_bl_lock(), thus setting the lock bit with
> __set_bit(), and then call hlist_bl_first() to get the first element,
> the returned pointer will be invalid. As LIST_BL_LOCKMASK is zero the
> least significant bit of the pointer will be 1.
> 
> I think for dm-snapshot to work using its own locking, and without
> list_bl complaining, the following is sufficient:
> 
> --- a/include/linux/list_bl.h
> +++ b/include/linux/list_bl.h
> @@ -25,7 +25,7 @@
>  #define LIST_BL_LOCKMASK       0UL
>  #endif
> 
> -#ifdef CONFIG_DEBUG_LIST
> +#if defined(CONFIG_DEBUG_LIST) && !defined(CONFIG_PREEMPT_RT_BASE)
>  #define LIST_BL_BUG_ON(x) BUG_ON(x)
>  #else
>  #define LIST_BL_BUG_ON(x)
> 
> Nikos

Yes - so, submit this.

Reviewed-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

Mikulas