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 14:27, 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 <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.

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