Re: [PATCH v6 07/10] mm: synchronize access to kmem_cache dying flag using a spinlock

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jun 04, 2019 at 07:44:51PM -0700, Roman Gushchin wrote:
> Currently the memcg_params.dying flag and the corresponding
> workqueue used for the asynchronous deactivation of kmem_caches
> is synchronized using the slab_mutex.
> 
> It makes impossible to check this flag from the irq context,
> which will be required in order to implement asynchronous release
> of kmem_caches.
> 
> So let's switch over to the irq-save flavor of the spinlock-based
> synchronization.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  mm/slab_common.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 09b26673b63f..2914a8f0aa85 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -130,6 +130,7 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,
>  #ifdef CONFIG_MEMCG_KMEM
>  
>  LIST_HEAD(slab_root_caches);
> +static DEFINE_SPINLOCK(memcg_kmem_wq_lock);
>  
>  void slab_init_memcg_params(struct kmem_cache *s)
>  {
> @@ -629,6 +630,7 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
>  	struct memcg_cache_array *arr;
>  	struct kmem_cache *s = NULL;
>  	char *cache_name;
> +	bool dying;
>  	int idx;
>  
>  	get_online_cpus();
> @@ -640,7 +642,13 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
>  	 * The memory cgroup could have been offlined while the cache
>  	 * creation work was pending.
>  	 */
> -	if (memcg->kmem_state != KMEM_ONLINE || root_cache->memcg_params.dying)
> +	if (memcg->kmem_state != KMEM_ONLINE)
> +		goto out_unlock;
> +
> +	spin_lock_irq(&memcg_kmem_wq_lock);
> +	dying = root_cache->memcg_params.dying;
> +	spin_unlock_irq(&memcg_kmem_wq_lock);
> +	if (dying)
>  		goto out_unlock;

What does this lock protect? The dying flag could get set right after
the unlock.