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 > 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 >>> } >>> >> >
