2019-11-12 16:18:03

by Mikulas Patocka

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

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 <[email protected]>

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
}

@@ -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
}

2019-11-13 10:32:21

by Nikos Tsironis

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 <[email protected]>
>
> 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
> }
>

2019-11-13 11:18:40

by Mikulas Patocka

Subject: Re: [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 <[email protected]>
> >
> > 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
> > }
> >
>

2019-11-13 11:22:39

by Mikulas Patocka

Subject: [PATCH RT 2/2 v3] list_bl: avoid BUG when the list is not locked

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 <[email protected]>

---
include/linux/list_bl.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-rt-devel/include/linux/list_bl.h
===================================================================
--- linux-rt-devel.orig/include/linux/list_bl.h 2019-11-13 12:15:50.000000000 +0100
+++ linux-rt-devel/include/linux/list_bl.h 2019-11-13 12:15:50.000000000 +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

2019-11-13 11:55:11

by Nikos Tsironis

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

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 <[email protected]>
>>>
>>> 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
>>> }
>>>
>>
>

2019-11-13 12:27:20

by Mikulas Patocka

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

On Wed, 13 Nov 2019, Nikos Tsironis wrote:

> On 11/13/19 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 <[email protected]>
> >>>
> >>> 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 <[email protected]>

Mikulas

2019-11-13 13:12:53

by Nikos Tsironis

Subject: [PATCH RT 2/2 v4] list_bl: avoid BUG when the list is not locked

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 deactivate the list debug checks for list_bl on
realtime kernels.

This patch is intended only for the realtime kernel patchset, not for
the upstream kernel.

Signed-off-by: Nikos Tsironis <[email protected]>
Reviewed-by: Mikulas Patocka <[email protected]>
---
include/linux/list_bl.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h
index da38433240f5..3585b2f6b948 100644
--- 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)
--
2.11.0