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

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