[PATCH] nvme: fix (S)RCU protection of nvme_ns_head list (alternate)

[Sagi Grimberg]

> This is an alternate fix, protecting the nvme_ns_head siblings list
> with both RCU and SRCU.

There is no need for another srcu protection.

> Use RCU in nvme_mpath_revalidate_paths(), which doesn't sleep.
> And add synchronize_srcu() in nvme_ns_remove(),
> since nvme_ns_head_submit_bio() protects the list with SRCU.

submit_bio is already fenced in steps before so I don't see
why there is a need for srcu protection here.

> Signed-off-by: Caleb Sander <csander at purestorage.com>
> ---
>   drivers/nvme/host/core.c      | 1 +
>   drivers/nvme/host/multipath.c | 2 ++
>   2 files changed, 3 insertions(+)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index da55ce45ac70..9b26d4781b32 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -4305,6 +4305,7 @@ static void nvme_ns_remove(struct nvme_ns *ns)
>   
>   	/* guarantee not available in head->list */
>   	synchronize_rcu();
> +	synchronize_srcu(&ns->head->srcu);

This can be removed.

>   
>   	if (!nvme_ns_head_multipath(ns->head))
>   		nvme_cdev_del(&ns->cdev, &ns->cdev_device);
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 93e2138a8b42..6d0aea477253 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -175,10 +175,12 @@ void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
>   	sector_t capacity = get_capacity(head->disk);
>   	int node;
>   
> +	rcu_read_lock();
>   	list_for_each_entry_rcu(ns, &head->list, siblings) {
>   		if (capacity != get_capacity(ns->disk))
>   			clear_bit(NVME_NS_READY, &ns->flags);
>   	}
> +	rcu_read_unlock();
>   

This looks fine.

>   	for_each_node(node)
>   		rcu_assign_pointer(head->current_path[node], NULL);