Re: [PATCH v2] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

[Waiman Long]

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 <longman@xxxxxxxxxx>
> > ---
> >   [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