A circular locking problem is reported by lockdep due to the following
circular locking dependency.
+--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
| |
+-----------------------------------------------------+
The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
happens in
kmem_cache_destroy(): cpus_read_lock(); mutex_lock(&slab_mutex);
==> sysfs_slab_unlink()
==> kobject_del()
==> kernfs_remove()
==> __kernfs_remove()
==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);
The backward kn->active ==> cpu_hotplug_lock dependency happens in
kernfs_fop_write_iter(): kernfs_get_active();
==> slab_attr_store()
==> cpu_partial_store()
==> flush_all(): cpus_read_lock()
One way to break this circular locking chain is to avoid holding
cpu_hotplug_lock and slab_mutex while deleting the kobject in
sysfs_slab_unlink() which should be equivalent to doing a write_lock
and write_unlock pair of the kn->active virtual lock.
Since the kobject structures are not protected by slab_mutex or the
cpu_hotplug_lock, we can certainly release those locks before doing
the delete operation.
Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
created kmem_cache_release() and call it outside the slab_mutex &
cpu_hotplug_lock critical sections.
Signed-off-by: Waiman Long <[email protected]>
---
[v2] Break kmem_cache_release() helper into 2 separate ones.
mm/slab_common.c | 54 +++++++++++++++++++++++++++++++++---------------
1 file changed, 37 insertions(+), 17 deletions(-)
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 17996649cfe3..7742d0446d8b 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -392,6 +392,36 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
}
EXPORT_SYMBOL(kmem_cache_create);
+#ifdef SLAB_SUPPORTS_SYSFS
+static void kmem_cache_workfn_release(struct kmem_cache *s)
+{
+ sysfs_slab_release(s);
+}
+#else
+static void kmem_cache_workfn_release(struct kmem_cache *s)
+{
+ slab_kmem_cache_release(s);
+}
+#endif
+
+/*
+ * For a given kmem_cache, kmem_cache_destroy() should only be called
+ * once or there will be a use-after-free problem. The actual deletion
+ * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
+ * protection. So they are now done without holding those locks.
+ */
+static void kmem_cache_release(struct kmem_cache *s)
+{
+#ifdef SLAB_SUPPORTS_SYSFS
+ sysfs_slab_unlink(s);
+#endif
+
+ if (s->flags & SLAB_TYPESAFE_BY_RCU)
+ schedule_work(&slab_caches_to_rcu_destroy_work);
+ else
+ kmem_cache_workfn_release(s);
+}
+
static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
{
LIST_HEAD(to_destroy);
@@ -418,11 +448,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
list_for_each_entry_safe(s, s2, &to_destroy, list) {
debugfs_slab_release(s);
kfence_shutdown_cache(s);
-#ifdef SLAB_SUPPORTS_SYSFS
- sysfs_slab_release(s);
-#else
- slab_kmem_cache_release(s);
-#endif
+ kmem_cache_workfn_release(s);
}
}
@@ -437,20 +463,10 @@ static int shutdown_cache(struct kmem_cache *s)
list_del(&s->list);
if (s->flags & SLAB_TYPESAFE_BY_RCU) {
-#ifdef SLAB_SUPPORTS_SYSFS
- sysfs_slab_unlink(s);
-#endif
list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
- schedule_work(&slab_caches_to_rcu_destroy_work);
} else {
kfence_shutdown_cache(s);
debugfs_slab_release(s);
-#ifdef SLAB_SUPPORTS_SYSFS
- sysfs_slab_unlink(s);
- sysfs_slab_release(s);
-#else
- slab_kmem_cache_release(s);
-#endif
}
return 0;
@@ -465,14 +481,16 @@ void slab_kmem_cache_release(struct kmem_cache *s)
void kmem_cache_destroy(struct kmem_cache *s)
{
+ int refcnt;
+
if (unlikely(!s) || !kasan_check_byte(s))
return;
cpus_read_lock();
mutex_lock(&slab_mutex);
- s->refcount--;
- if (s->refcount)
+ refcnt = --s->refcount;
+ if (refcnt)
goto out_unlock;
WARN(shutdown_cache(s),
@@ -481,6 +499,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
out_unlock:
mutex_unlock(&slab_mutex);
cpus_read_unlock();
+ if (!refcnt)
+ kmem_cache_release(s);
}
EXPORT_SYMBOL(kmem_cache_destroy);
--
2.31.1
On 8/10/22 14:10, Roman Gushchin wrote:
> On Wed, Aug 10, 2022 at 12:49:46PM -0400, Waiman Long wrote:
>> A circular locking problem is reported by lockdep due to the following
>> circular locking dependency.
>>
>> +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
>> | |
>> +-----------------------------------------------------+
>>
>> The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
>> happens in
>>
>> kmem_cache_destroy(): cpus_read_lock(); mutex_lock(&slab_mutex);
>> ==> sysfs_slab_unlink()
>> ==> kobject_del()
>> ==> kernfs_remove()
>> ==> __kernfs_remove()
>> ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);
>>
>> The backward kn->active ==> cpu_hotplug_lock dependency happens in
>>
>> kernfs_fop_write_iter(): kernfs_get_active();
>> ==> slab_attr_store()
>> ==> cpu_partial_store()
>> ==> flush_all(): cpus_read_lock()
>>
>> One way to break this circular locking chain is to avoid holding
>> cpu_hotplug_lock and slab_mutex while deleting the kobject in
>> sysfs_slab_unlink() which should be equivalent to doing a write_lock
>> and write_unlock pair of the kn->active virtual lock.
>>
>> Since the kobject structures are not protected by slab_mutex or the
>> cpu_hotplug_lock, we can certainly release those locks before doing
>> the delete operation.
>>
>> Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
>> created kmem_cache_release() and call it outside the slab_mutex &
>> cpu_hotplug_lock critical sections.
>>
>> Signed-off-by: Waiman Long <[email protected]>
>> ---
>> [v2] Break kmem_cache_release() helper into 2 separate ones.
>>
>> mm/slab_common.c | 54 +++++++++++++++++++++++++++++++++---------------
>> 1 file changed, 37 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 17996649cfe3..7742d0446d8b 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -392,6 +392,36 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
>> }
>> EXPORT_SYMBOL(kmem_cache_create);
>>
>> +#ifdef SLAB_SUPPORTS_SYSFS
>> +static void kmem_cache_workfn_release(struct kmem_cache *s)
>> +{
>> + sysfs_slab_release(s);
>> +}
>> +#else
>> +static void kmem_cache_workfn_release(struct kmem_cache *s)
>> +{
>> + slab_kmem_cache_release(s);
>> +}
>> +#endif
>> +
>> +/*
>> + * For a given kmem_cache, kmem_cache_destroy() should only be called
>> + * once or there will be a use-after-free problem. The actual deletion
>> + * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
>> + * protection. So they are now done without holding those locks.
>> + */
>> +static void kmem_cache_release(struct kmem_cache *s)
>> +{
>> +#ifdef SLAB_SUPPORTS_SYSFS
>> + sysfs_slab_unlink(s);
>> +#endif
>> +
>> + if (s->flags & SLAB_TYPESAFE_BY_RCU)
>> + schedule_work(&slab_caches_to_rcu_destroy_work);
>> + else
>> + kmem_cache_workfn_release(s);
>> +}
>> +
>> static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>> {
>> LIST_HEAD(to_destroy);
>> @@ -418,11 +448,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>> list_for_each_entry_safe(s, s2, &to_destroy, list) {
>> debugfs_slab_release(s);
>> kfence_shutdown_cache(s);
>> -#ifdef SLAB_SUPPORTS_SYSFS
>> - sysfs_slab_release(s);
>> -#else
>> - slab_kmem_cache_release(s);
>> -#endif
>> + kmem_cache_workfn_release(s);
>> }
>> }
>>
>> @@ -437,20 +463,10 @@ static int shutdown_cache(struct kmem_cache *s)
>> list_del(&s->list);
>>
>> if (s->flags & SLAB_TYPESAFE_BY_RCU) {
>> -#ifdef SLAB_SUPPORTS_SYSFS
>> - sysfs_slab_unlink(s);
>> -#endif
>> list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
>> - schedule_work(&slab_caches_to_rcu_destroy_work);
> Hi Waiman!
>
> This version is much more readable, thank you!
>
> But can we, please, leave this schedule_work(&slab_caches_to_rcu_destroy_work)
> call here? I don't see a good reason to move it, do I miss something?
> It's nice to have list_add_tail() and schedule_work() calls nearby, so
> it's obvious we can't miss the latter.
The reason that I need to move out schedule_work() as well is to make
sure that sysfs_slab_unlink() is called before sysfs_slab_release(). I
can't guarantee that if I do schedule_work() first. On the other hand,
moving sysfs_slab_unlink() into kmem_cache_workfn_release() introduces
unknown delay of when the sysfs file will be removed. I can add some
comment to make it more clear.
Please let me know if you have a better idea of dealing with this issue.
Thanks,
Longman
On Wed, Aug 10, 2022 at 12:49:46PM -0400, Waiman Long wrote:
> A circular locking problem is reported by lockdep due to the following
> circular locking dependency.
>
> +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
> | |
> +-----------------------------------------------------+
>
> The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
> happens in
>
> kmem_cache_destroy(): cpus_read_lock(); mutex_lock(&slab_mutex);
> ==> sysfs_slab_unlink()
> ==> kobject_del()
> ==> kernfs_remove()
> ==> __kernfs_remove()
> ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);
>
> The backward kn->active ==> cpu_hotplug_lock dependency happens in
>
> kernfs_fop_write_iter(): kernfs_get_active();
> ==> slab_attr_store()
> ==> cpu_partial_store()
> ==> flush_all(): cpus_read_lock()
>
> One way to break this circular locking chain is to avoid holding
> cpu_hotplug_lock and slab_mutex while deleting the kobject in
> sysfs_slab_unlink() which should be equivalent to doing a write_lock
> and write_unlock pair of the kn->active virtual lock.
>
> Since the kobject structures are not protected by slab_mutex or the
> cpu_hotplug_lock, we can certainly release those locks before doing
> the delete operation.
>
> Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
> created kmem_cache_release() and call it outside the slab_mutex &
> cpu_hotplug_lock critical sections.
>
> Signed-off-by: Waiman Long <[email protected]>
> ---
> [v2] Break kmem_cache_release() helper into 2 separate ones.
>
> mm/slab_common.c | 54 +++++++++++++++++++++++++++++++++---------------
> 1 file changed, 37 insertions(+), 17 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 17996649cfe3..7742d0446d8b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -392,6 +392,36 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
> }
> EXPORT_SYMBOL(kmem_cache_create);
>
> +#ifdef SLAB_SUPPORTS_SYSFS
> +static void kmem_cache_workfn_release(struct kmem_cache *s)
> +{
> + sysfs_slab_release(s);
> +}
> +#else
> +static void kmem_cache_workfn_release(struct kmem_cache *s)
> +{
> + slab_kmem_cache_release(s);
> +}
> +#endif
> +
> +/*
> + * For a given kmem_cache, kmem_cache_destroy() should only be called
> + * once or there will be a use-after-free problem. The actual deletion
> + * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
> + * protection. So they are now done without holding those locks.
> + */
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> +#ifdef SLAB_SUPPORTS_SYSFS
> + sysfs_slab_unlink(s);
> +#endif
> +
> + if (s->flags & SLAB_TYPESAFE_BY_RCU)
> + schedule_work(&slab_caches_to_rcu_destroy_work);
> + else
> + kmem_cache_workfn_release(s);
> +}
> +
> static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> {
> LIST_HEAD(to_destroy);
> @@ -418,11 +448,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> list_for_each_entry_safe(s, s2, &to_destroy, list) {
> debugfs_slab_release(s);
> kfence_shutdown_cache(s);
> -#ifdef SLAB_SUPPORTS_SYSFS
> - sysfs_slab_release(s);
> -#else
> - slab_kmem_cache_release(s);
> -#endif
> + kmem_cache_workfn_release(s);
> }
> }
>
> @@ -437,20 +463,10 @@ static int shutdown_cache(struct kmem_cache *s)
> list_del(&s->list);
>
> if (s->flags & SLAB_TYPESAFE_BY_RCU) {
> -#ifdef SLAB_SUPPORTS_SYSFS
> - sysfs_slab_unlink(s);
> -#endif
> list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
> - schedule_work(&slab_caches_to_rcu_destroy_work);
Hi Waiman!
This version is much more readable, thank you!
But can we, please, leave this schedule_work(&slab_caches_to_rcu_destroy_work)
call here? I don't see a good reason to move it, do I miss something?
It's nice to have list_add_tail() and schedule_work() calls nearby, so
it's obvious we can't miss the latter.
Thanks!
On 8/10/22 14:27, Waiman Long wrote:
> On 8/10/22 14:10, Roman Gushchin wrote:
>> On Wed, Aug 10, 2022 at 12:49:46PM -0400, Waiman Long wrote:
>>> A circular locking problem is reported by lockdep due to the following
>>> circular locking dependency.
>>>
>>> +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
>>> | |
>>> +-----------------------------------------------------+
>>>
>>> The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
>>> happens in
>>>
>>> kmem_cache_destroy(): cpus_read_lock(); mutex_lock(&slab_mutex);
>>> ==> sysfs_slab_unlink()
>>> ==> kobject_del()
>>> ==> kernfs_remove()
>>> ==> __kernfs_remove()
>>> ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);
>>>
>>> The backward kn->active ==> cpu_hotplug_lock dependency happens in
>>>
>>> kernfs_fop_write_iter(): kernfs_get_active();
>>> ==> slab_attr_store()
>>> ==> cpu_partial_store()
>>> ==> flush_all(): cpus_read_lock()
>>>
>>> One way to break this circular locking chain is to avoid holding
>>> cpu_hotplug_lock and slab_mutex while deleting the kobject in
>>> sysfs_slab_unlink() which should be equivalent to doing a write_lock
>>> and write_unlock pair of the kn->active virtual lock.
>>>
>>> Since the kobject structures are not protected by slab_mutex or the
>>> cpu_hotplug_lock, we can certainly release those locks before doing
>>> the delete operation.
>>>
>>> Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
>>> created kmem_cache_release() and call it outside the slab_mutex &
>>> cpu_hotplug_lock critical sections.
>>>
>>> Signed-off-by: Waiman Long <[email protected]>
>>> ---
>>> [v2] Break kmem_cache_release() helper into 2 separate ones.
>>>
>>> mm/slab_common.c | 54
>>> +++++++++++++++++++++++++++++++++---------------
>>> 1 file changed, 37 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>>> index 17996649cfe3..7742d0446d8b 100644
>>> --- a/mm/slab_common.c
>>> +++ b/mm/slab_common.c
>>> @@ -392,6 +392,36 @@ kmem_cache_create(const char *name, unsigned
>>> int size, unsigned int align,
>>> }
>>> EXPORT_SYMBOL(kmem_cache_create);
>>> +#ifdef SLAB_SUPPORTS_SYSFS
>>> +static void kmem_cache_workfn_release(struct kmem_cache *s)
>>> +{
>>> + sysfs_slab_release(s);
>>> +}
>>> +#else
>>> +static void kmem_cache_workfn_release(struct kmem_cache *s)
>>> +{
>>> + slab_kmem_cache_release(s);
>>> +}
>>> +#endif
>>> +
>>> +/*
>>> + * For a given kmem_cache, kmem_cache_destroy() should only be called
>>> + * once or there will be a use-after-free problem. The actual deletion
>>> + * and release of the kobject does not need slab_mutex or
>>> cpu_hotplug_lock
>>> + * protection. So they are now done without holding those locks.
>>> + */
>>> +static void kmem_cache_release(struct kmem_cache *s)
>>> +{
>>> +#ifdef SLAB_SUPPORTS_SYSFS
>>> + sysfs_slab_unlink(s);
>>> +#endif
>>> +
>>> + if (s->flags & SLAB_TYPESAFE_BY_RCU)
>>> + schedule_work(&slab_caches_to_rcu_destroy_work);
>>> + else
>>> + kmem_cache_workfn_release(s);
>>> +}
>>> +
>>> static void slab_caches_to_rcu_destroy_workfn(struct work_struct
>>> *work)
>>> {
>>> LIST_HEAD(to_destroy);
>>> @@ -418,11 +448,7 @@ static void
>>> slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>>> list_for_each_entry_safe(s, s2, &to_destroy, list) {
>>> debugfs_slab_release(s);
>>> kfence_shutdown_cache(s);
>>> -#ifdef SLAB_SUPPORTS_SYSFS
>>> - sysfs_slab_release(s);
>>> -#else
>>> - slab_kmem_cache_release(s);
>>> -#endif
>>> + kmem_cache_workfn_release(s);
>>> }
>>> }
>>> @@ -437,20 +463,10 @@ static int shutdown_cache(struct kmem_cache *s)
>>> list_del(&s->list);
>>> if (s->flags & SLAB_TYPESAFE_BY_RCU) {
>>> -#ifdef SLAB_SUPPORTS_SYSFS
>>> - sysfs_slab_unlink(s);
>>> -#endif
>>> list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
>>> - schedule_work(&slab_caches_to_rcu_destroy_work);
>> Hi Waiman!
>>
>> This version is much more readable, thank you!
>>
>> But can we, please, leave this
>> schedule_work(&slab_caches_to_rcu_destroy_work)
>> call here? I don't see a good reason to move it, do I miss something?
>> It's nice to have list_add_tail() and schedule_work() calls nearby, so
>> it's obvious we can't miss the latter.
>
> The reason that I need to move out schedule_work() as well is to make
> sure that sysfs_slab_unlink() is called before sysfs_slab_release(). I
> can't guarantee that if I do schedule_work() first. On the other hand,
> moving sysfs_slab_unlink() into kmem_cache_workfn_release() introduces
> unknown delay of when the sysfs file will be removed. I can add some
> comment to make it more clear.
OK, I just realize that the current patch doesn't have the ordering
guarantee either if another kmem_cache_destroy() is happening in
parallel. I will have to push sysfs_slab_unlink() into
kmem_cache_workfn_release() and tolerate some delay in the disappearance
of the sysfs files. Now I can move schedule_work() back to after
list_add_tail().
Cheers,
Longman