Re: [PATCH 3/5] nvme: track shared namespaces

On Thu, Nov 09, 2017 at 02:59:50PM +0200, Sagi Grimberg wrote:

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To allow lockless path lookup the list of nvme_ns structures per
nvme_ns_head is protected by SRCU, which requires freeing the nvme_ns
structure through call_srcu.

Can you remind me why isn't rcu sufficient? Can looking up a
path (ns from head->list) block?

blk_mq_make_request can block.

Oh I see, so can you explain why srcu should cover direct_make_request?

What if we were to do:
--
	rcu_read_lock();
	ns = nvme_find_path(head);
	rcu_read_unlock();
	if (likely(ns)) {
		...
--

That way we'd have to take and release a namespace reference for every I/O.