linux-kernel - Re: [PATCH RT 2/2 v2] list_bl: avoid BUG when the list is not locked

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Date:   Wed, 13 Nov 2019 12:29:08 +0200
From:   Nikos Tsironis <ntsironis@...ikto.com>
To:     Mikulas Patocka <mpatocka@...hat.com>, tglx@...utronix.de,
        linux-rt-users@...r.kernel.org
Cc:     Mike Snitzer <msnitzer@...hat.com>, Scott Wood <swood@...hat.com>,
        Ilias Tsitsimpis <iliastsi@...ikto.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Daniel Wagner <dwagner@...e.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH RT 2/2 v2] list_bl: avoid BUG when the list is not locked

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@...hat.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
>  }
>