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

> @@ -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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