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
