Re: [PATCH v2 3/4] mm/slub: Fix another circular locking dependency in slab_attr_store()

[Qian Cai]

> On Apr 27, 2020, at 7:56 PM, Waiman Long <longman@xxxxxxxxxx> wrote:
> 
> It turns out that switching from slab_mutex to memcg_cache_ids_sem in
> slab_attr_store() does not completely eliminate circular locking dependency
> as shown by the following lockdep splat when the system is shut down:
> 
> [ 2095.079697] Chain exists of:
> [ 2095.079697]   kn->count#278 --> memcg_cache_ids_sem --> slab_mutex
> [ 2095.079697]
> [ 2095.090278]  Possible unsafe locking scenario:
> [ 2095.090278]
> [ 2095.096227]        CPU0                    CPU1
> [ 2095.100779]        ----                    ----
> [ 2095.105331]   lock(slab_mutex);
> [ 2095.108486]                                lock(memcg_cache_ids_sem);
> [ 2095.114961]                                lock(slab_mutex);
> [ 2095.120649]   lock(kn->count#278);
> [ 2095.124068]
> [ 2095.124068]  *** DEADLOCK ***

Can you show the full splat?

> 
> To eliminate this possibility, we have to use trylock to acquire
> memcg_cache_ids_sem. Unlikely slab_mutex which can be acquired in
> many places, the memcg_cache_ids_sem write lock is only acquired
> in memcg_alloc_cache_id() to double the size of memcg_nr_cache_ids.
> So the chance of successive calls to memcg_alloc_cache_id() within
> a short time is pretty low. As a result, we can retry the read lock
> acquisition a few times if the first attempt fails.
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

The code looks a bit hacky and probably not that robust. Since it is the shutdown path which is not all that important without lockdep, maybe you could drop this single patch for now until there is a better solution?

> ---
> include/linux/memcontrol.h |  1 +
> mm/memcontrol.c            |  5 +++++
> mm/slub.c                  | 25 +++++++++++++++++++++++--
> 3 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index d275c72c4f8e..9285f14965b1 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1379,6 +1379,7 @@ extern struct workqueue_struct *memcg_kmem_cache_wq;
> extern int memcg_nr_cache_ids;
> void memcg_get_cache_ids(void);
> void memcg_put_cache_ids(void);
> +int  memcg_tryget_cache_ids(void);
> 
> /*
>  * Helper macro to loop through all memcg-specific caches. Callers must still
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5beea03dd58a..9fa8535ff72a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -279,6 +279,11 @@ void memcg_get_cache_ids(void)
>    down_read(&memcg_cache_ids_sem);
> }
> 
> +int memcg_tryget_cache_ids(void)
> +{
> +    return down_read_trylock(&memcg_cache_ids_sem);
> +}
> +
> void memcg_put_cache_ids(void)
> {
>    up_read(&memcg_cache_ids_sem);
> diff --git a/mm/slub.c b/mm/slub.c
> index 44cb5215c17f..cf2114ca27f7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -34,6 +34,7 @@
> #include <linux/prefetch.h>
> #include <linux/memcontrol.h>
> #include <linux/random.h>
> +#include <linux/delay.h>
> 
> #include <trace/events/kmem.h>
> 
> @@ -5572,6 +5573,7 @@ static ssize_t slab_attr_store(struct kobject *kobj,
>        !list_empty(&s->memcg_params.children)) {
>        struct kmem_cache *c, **pcaches;
>        int idx, max, cnt = 0;
> +        int retries = 3;
>        size_t size, old = s->max_attr_size;
>        struct memcg_cache_array *arr;
> 
> @@ -5585,9 +5587,28 @@ static ssize_t slab_attr_store(struct kobject *kobj,
>            old = cmpxchg(&s->max_attr_size, size, len);
>        } while (old != size);
> 
> -        memcg_get_cache_ids();
> -        max = memcg_nr_cache_ids;
> +        /*
> +         * To avoid the following circular lock chain
> +         *
> +         *   kn->count#278 --> memcg_cache_ids_sem --> slab_mutex
> +         *
> +         * We need to use trylock to acquire memcg_cache_ids_sem.
> +         *
> +         * Since the write lock is acquired only in
> +         * memcg_alloc_cache_id() to double the size of
> +         * memcg_nr_cache_ids. The chance of successive
> +         * memcg_alloc_cache_id() calls within a short time is
> +         * very low except at the beginning where the number of
> +         * memory cgroups is low. So we retry a few times to get
> +         * the memcg_cache_ids_sem read lock.
> +         */
> +        while (!memcg_tryget_cache_ids()) {
> +            if (retries-- <= 0)
> +                return -EBUSY;
> +            msleep(100);
> +        }
> 
> +        max = memcg_nr_cache_ids;
>        pcaches = kmalloc_array(max, sizeof(void *), GFP_KERNEL);
>        if (!pcaches) {
>            memcg_put_cache_ids();