From: Roland Dreier <roland@purestorage.com>
Date: Tue, 29 Aug 2017 10:33:44 -0700
Subject: nvme-fabrics: Convert nvmf_transports_mutex to an rwsem
Patch-mainline: v4.14-rc1
Git-commit: 489beb91e66a237254e23ac1a0fe1beac23d87c5
References: FATE#323952, FATE#322506

The mutex protects against the list of transports changing while a
controller is being created, but using a plain old mutex means that it
also serializes controller creation.  This unnecessarily slows down
creating multiple controllers - for example for the RDMA transport,
creating a controller involves establishing one connection for every IO
queue, which involves even more network/software round trips, so the
delay can become significant.

The simplest way to fix this is to change the mutex to an rwsem and only
hold it for writing when the list is being mutated.  Since we can take
the rwsem for reading while creating a controller, we can create multiple
controllers in parallel.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Johannes Thumshirn <jthumshirn@suse.de>
---
 drivers/nvme/host/fabrics.c |   20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

--- a/drivers/nvme/host/fabrics.c
+++ b/drivers/nvme/host/fabrics.c
@@ -22,7 +22,7 @@
 #include "fabrics.h"
 
 static LIST_HEAD(nvmf_transports);
-static DEFINE_MUTEX(nvmf_transports_mutex);
+static DECLARE_RWSEM(nvmf_transports_rwsem);
 
 static LIST_HEAD(nvmf_hosts);
 static DEFINE_MUTEX(nvmf_hosts_mutex);
@@ -495,9 +495,9 @@ int nvmf_register_transport(struct nvmf_
 	if (!ops->create_ctrl)
 		return -EINVAL;
 
-	mutex_lock(&nvmf_transports_mutex);
+	down_write(&nvmf_transports_rwsem);
 	list_add_tail(&ops->entry, &nvmf_transports);
-	mutex_unlock(&nvmf_transports_mutex);
+	up_write(&nvmf_transports_rwsem);
 
 	return 0;
 }
@@ -514,9 +514,9 @@ EXPORT_SYMBOL_GPL(nvmf_register_transpor
  */
 void nvmf_unregister_transport(struct nvmf_transport_ops *ops)
 {
-	mutex_lock(&nvmf_transports_mutex);
+	down_write(&nvmf_transports_rwsem);
 	list_del(&ops->entry);
-	mutex_unlock(&nvmf_transports_mutex);
+	up_write(&nvmf_transports_rwsem);
 }
 EXPORT_SYMBOL_GPL(nvmf_unregister_transport);
 
@@ -525,7 +525,7 @@ static struct nvmf_transport_ops *nvmf_l
 {
 	struct nvmf_transport_ops *ops;
 
-	lockdep_assert_held(&nvmf_transports_mutex);
+	lockdep_assert_held(&nvmf_transports_rwsem);
 
 	list_for_each_entry(ops, &nvmf_transports, entry) {
 		if (strcmp(ops->name, opts->transport) == 0)
@@ -851,7 +851,7 @@ nvmf_create_ctrl(struct device *dev, con
 		goto out_free_opts;
 	opts->mask &= ~NVMF_REQUIRED_OPTS;
 
-	mutex_lock(&nvmf_transports_mutex);
+	down_read(&nvmf_transports_rwsem);
 	ops = nvmf_lookup_transport(opts);
 	if (!ops) {
 		pr_info("no handler found for transport %s.\n",
@@ -878,16 +878,16 @@ nvmf_create_ctrl(struct device *dev, con
 		dev_warn(ctrl->device,
 			"controller returned incorrect NQN: \"%s\".\n",
 			ctrl->subnqn);
-		mutex_unlock(&nvmf_transports_mutex);
+		up_read(&nvmf_transports_rwsem);
 		ctrl->ops->delete_ctrl(ctrl);
 		return ERR_PTR(-EINVAL);
 	}
 
-	mutex_unlock(&nvmf_transports_mutex);
+	up_read(&nvmf_transports_rwsem);
 	return ctrl;
 
 out_unlock:
-	mutex_unlock(&nvmf_transports_mutex);
+	up_read(&nvmf_transports_rwsem);
 out_free_opts:
 	nvmf_free_options(opts);
 	return ERR_PTR(ret);