| Message ID | 20220810164946.148634-1-longman@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock | expand |
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@redhat.com> > --- > [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 14:10, 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@redhat.com> >> --- >> [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 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@redhat.com> >>> --- >>> [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
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);
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@redhat.com> --- [v2] Break kmem_cache_release() helper into 2 separate ones. mm/slab_common.c | 54 +++++++++++++++++++++++++++++++++--------------- 1 file changed, 37 insertions(+), 17 deletions(-)