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

[Waiman Long]

On 8/9/22 18:25, Roman Gushchin wrote:
> On Tue, Aug 09, 2022 at 04:59:01PM -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#126 --+
> >    |                                                         |
> >    +---------------------------------------------------------+
> >
> > 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>
> > ---
> >   mm/slab_common.c | 48 +++++++++++++++++++++++++++++++-----------------
> >   1 file changed, 31 insertions(+), 17 deletions(-)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 17996649cfe3..9274fb03563e 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -392,6 +392,30 @@ 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.
> > + */
> > +static void kmem_cache_release(struct kmem_cache *s, bool workfn)
> > +{
> > +	if (!workfn)
> > +		sysfs_slab_unlink(s);
> > +
> > +	if (workfn || !(s->flags & SLAB_TYPESAFE_BY_RCU))
> > +		sysfs_slab_release(s);
> > +	else
> > +		schedule_work(&slab_caches_to_rcu_destroy_work);
> > +}
> > +#else
> > +static inline void kmem_cache_release(struct kmem_cache *s, bool workfn)
> > +{
> > +	slab_kmem_cache_release(s);
> > +}
> > +#endif
> > +
> >   static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> >   {
> >   	LIST_HEAD(to_destroy);
> > @@ -418,11 +442,7 @@ 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, true);
> Hi Waiman!
>
> As I understand, with SLAB_SUPPORTS_SYSFS kmem_cache_release() can effectively call
> into itself: first it's called with workfn == false from shutdown_cache() and
> then optionally it's scheduled to call itself from a work context with
> workfn == true just to call sysfs_slab_release(). Is it right?
>
> If !SLAB_SUPPORTS_SYSFS, shutdown_cache() optionally adds kmem_cache to the
> slab_caches_to_rcu_destroy list and calls kmem_cache_release(s, false) ==
> slab_kmem_cache_release(). How it's then removed from the list?
>
> Overall the patch is a bit hard to follow (not like this code was easy to read
> before, so can't blame the patch). But I wonder if it will make things simpler
> to decouple kmem_cache_release(workfn == true) and kmem_cache_release(workfn == false)
> into 2 different helpers? Or at least add a bold comment on how things are supposed
> to work.
>
> Thanks!

You are right. I agree that it can be hard to read. Simpler is always 
better. Will post a v2 with the change suggested.

Thanks,
Longman