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


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@redhat.com>
> >
> > 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
> > }
> >
>

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