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. There will be a slight delay
in the deletion of sysfs files if kmem_cache_release() is called
indirectly from a work function.
Signed-off-by: Waiman Long <[email protected]>
---
[v3] Move sysfs_slab_unlink() out to kmem_cache_release() and move
schedule_work() back right after list_add_tail().
mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
1 file changed, 29 insertions(+), 16 deletions(-)
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 17996649cfe3..07b948288f84 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -392,6 +392,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
}
EXPORT_SYMBOL(kmem_cache_create);
+#ifdef SLAB_SUPPORTS_SYSFS
+/*
+ * 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.
+ *
+ * Note that there will be a slight delay in the deletion of sysfs files
+ * if kmem_cache_release() is called indrectly from a work function.
+ */
+static void kmem_cache_release(struct kmem_cache *s)
+{
+ sysfs_slab_unlink(s);
+ sysfs_slab_release(s);
+}
+#else
+static void kmem_cache_release(struct kmem_cache *s)
+{
+ slab_kmem_cache_release(s);
+}
+#endif
+
static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
{
LIST_HEAD(to_destroy);
@@ -418,11 +440,7 @@ 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_release(s);
}
}
@@ -437,20 +455,11 @@ 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 +474,16 @@ 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 +492,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
out_unlock:
mutex_unlock(&slab_mutex);
cpus_read_unlock();
+ if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
+ kmem_cache_release(s);
}
EXPORT_SYMBOL(kmem_cache_destroy);
--
2.31.1
On Fri, Aug 12, 2022 at 02:30:33PM -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, ...);
Maybe you mean this?
/* but everyone should wait for draining */
wait_event(root->deactivate_waitq,
atomic_read(&kn->active) == KN_DEACTIVATED_BIAS);
> 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. There will be a slight delay
> in the deletion of sysfs files if kmem_cache_release() is called
> indirectly from a work function.
>
> Signed-off-by: Waiman Long <[email protected]>
> ---
>
> [v3] Move sysfs_slab_unlink() out to kmem_cache_release() and move
> schedule_work() back right after list_add_tail().
>
> mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
> 1 file changed, 29 insertions(+), 16 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 17996649cfe3..07b948288f84 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -392,6 +392,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
> }
> EXPORT_SYMBOL(kmem_cache_create);
>
> +#ifdef SLAB_SUPPORTS_SYSFS
> +/*
> + * 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.
> + *
> + * Note that there will be a slight delay in the deletion of sysfs files
> + * if kmem_cache_release() is called indrectly from a work function.
> + */
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> + sysfs_slab_unlink(s);
> + sysfs_slab_release(s);
> +}
> +#else
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> + slab_kmem_cache_release(s);
> +}
> +#endif
> +
> static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> {
> LIST_HEAD(to_destroy);
> @@ -418,11 +440,7 @@ 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_release(s);
> }
> }
>
> @@ -437,20 +455,11 @@ 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 +474,16 @@ 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 +492,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
> out_unlock:
> mutex_unlock(&slab_mutex);
> cpus_read_unlock();
> + if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
> + kmem_cache_release(s);
> }
> EXPORT_SYMBOL(kmem_cache_destroy);
>
> --
> 2.31.1
>
little bit complicated but looks good to me.
Thank you for fixing this.
Reviewed-by: Hyeonggon Yoo <[email protected]>
--
Thanks,
Hyeonggon
On Fri, Aug 12, 2022 at 02:30:33PM -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. There will be a slight delay
> in the deletion of sysfs files if kmem_cache_release() is called
> indirectly from a work function.
>
> Signed-off-by: Waiman Long <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>
Thank you, Waiman!
On Fri, 12 Aug 2022, 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. There will be a slight delay
> in the deletion of sysfs files if kmem_cache_release() is called
> indirectly from a work function.
>
> Signed-off-by: Waiman Long <[email protected]>
Acked-by: David Rientjes <[email protected]>
On 8/13/22 11:00, Hyeonggon Yoo wrote:
> On Fri, Aug 12, 2022 at 02:30:33PM -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, ...);
> Maybe you mean this?
>
> /* but everyone should wait for draining */
> wait_event(root->deactivate_waitq,
> atomic_read(&kn->active) == KN_DEACTIVATED_BIAS);
>
This is part of the kernfs_drain() operation. However, I am focusing on
the behavior of the pseudo lock that causes the lockdep splat in the
first place. That is why I am showcasing the rwsem_acquire() call here.
>> 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. There will be a slight delay
>> in the deletion of sysfs files if kmem_cache_release() is called
>> indirectly from a work function.
>>
>> Signed-off-by: Waiman Long <[email protected]>
>> ---
>>
>> [v3] Move sysfs_slab_unlink() out to kmem_cache_release() and move
>> schedule_work() back right after list_add_tail().
>>
>> mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
>> 1 file changed, 29 insertions(+), 16 deletions(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 17996649cfe3..07b948288f84 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -392,6 +392,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
>> }
>> EXPORT_SYMBOL(kmem_cache_create);
>>
>> +#ifdef SLAB_SUPPORTS_SYSFS
>> +/*
>> + * 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.
>> + *
>> + * Note that there will be a slight delay in the deletion of sysfs files
>> + * if kmem_cache_release() is called indrectly from a work function.
>> + */
>> +static void kmem_cache_release(struct kmem_cache *s)
>> +{
>> + sysfs_slab_unlink(s);
>> + sysfs_slab_release(s);
>> +}
>> +#else
>> +static void kmem_cache_release(struct kmem_cache *s)
>> +{
>> + slab_kmem_cache_release(s);
>> +}
>> +#endif
>> +
>> static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>> {
>> LIST_HEAD(to_destroy);
>> @@ -418,11 +440,7 @@ 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_release(s);
>> }
>> }
>>
>> @@ -437,20 +455,11 @@ 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 +474,16 @@ 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 +492,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>> out_unlock:
>> mutex_unlock(&slab_mutex);
>> cpus_read_unlock();
>> + if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
>> + kmem_cache_release(s);
>> }
>> EXPORT_SYMBOL(kmem_cache_destroy);
>>
>> --
>> 2.31.1
>>
> little bit complicated but looks good to me.
> Thank you for fixing this.
>
> Reviewed-by: Hyeonggon Yoo <[email protected]>
>
Thanks for the review.
Cheers,
Longman
On 8/13/22 14:25, Roman Gushchin wrote:
> On Fri, Aug 12, 2022 at 02:30:33PM -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. There will be a slight delay
>> in the deletion of sysfs files if kmem_cache_release() is called
>> indirectly from a work function.
>>
>> Signed-off-by: Waiman Long <[email protected]>
> Reviewed-by: Roman Gushchin <[email protected]>
>
> Thank you, Waiman!
>
Thanks for your suggestions that make this patch better.
Cheers,
Longman
On 8/12/22 20:30, 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. There will be a slight delay
> in the deletion of sysfs files if kmem_cache_release() is called
> indirectly from a work function.
>
> Signed-off-by: Waiman Long <[email protected]>
Thanks, added to slab.git for-6.0/fixes
> ---
>
> [v3] Move sysfs_slab_unlink() out to kmem_cache_release() and move
> schedule_work() back right after list_add_tail().
>
> mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
> 1 file changed, 29 insertions(+), 16 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 17996649cfe3..07b948288f84 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -392,6 +392,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
> }
> EXPORT_SYMBOL(kmem_cache_create);
>
> +#ifdef SLAB_SUPPORTS_SYSFS
> +/*
> + * 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.
> + *
> + * Note that there will be a slight delay in the deletion of sysfs files
> + * if kmem_cache_release() is called indrectly from a work function.
> + */
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> + sysfs_slab_unlink(s);
> + sysfs_slab_release(s);
> +}
> +#else
> +static void kmem_cache_release(struct kmem_cache *s)
> +{
> + slab_kmem_cache_release(s);
> +}
> +#endif
> +
> static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> {
> LIST_HEAD(to_destroy);
> @@ -418,11 +440,7 @@ 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_release(s);
> }
> }
>
> @@ -437,20 +455,11 @@ 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 +474,16 @@ 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 +492,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
> out_unlock:
> mutex_unlock(&slab_mutex);
> cpus_read_unlock();
> + if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
> + kmem_cache_release(s);
> }
> EXPORT_SYMBOL(kmem_cache_destroy);
>
On 8/23/22 07:33, Vlastimil Babka wrote:
> On 8/12/22 20:30, 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. There will be a slight delay
>> in the deletion of sysfs files if kmem_cache_release() is called
>> indirectly from a work function.
>>
>> Signed-off-by: Waiman Long <[email protected]>
> Thanks, added to slab.git for-6.0/fixes
Thanks for taking it.
Cheers,
Longman