tcm_vhost: Use llist for cmd completion list

This drops the cmd completion list spin lock and makes the cmd
completion queue lock-less.
Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/vhost/tcm_vhost.c

@@ -47,6 +47,7 @@
 #include <linux/vhost.h>
 #include <linux/virtio_net.h> /* TODO vhost.h currently depends on this */
 #include <linux/virtio_scsi.h>
+#include <linux/llist.h>
 
 #include "vhost.c"
 #include "vhost.h"
@@ -64,8 +65,7 @@ struct vhost_scsi {
 	struct vhost_virtqueue vqs[3];
 
 	struct vhost_work vs_completion_work; /* cmd completion work item */
-	struct list_head vs_completion_list;  /* cmd completion queue */
-	spinlock_t vs_completion_lock;        /* protects s_completion_list */
+	struct llist_head vs_completion_list; /* cmd completion queue */
 };
 
 /* Local pointer to allocated TCM configfs fabric module */
@@ -301,9 +301,7 @@ static void vhost_scsi_complete_cmd(struct tcm_vhost_cmd *tv_cmd)
 {
 	struct vhost_scsi *vs = tv_cmd->tvc_vhost;
 
-	spin_lock_bh(&vs->vs_completion_lock);
-	list_add_tail(&tv_cmd->tvc_completion_list, &vs->vs_completion_list);
-	spin_unlock_bh(&vs->vs_completion_lock);
+	llist_add(&tv_cmd->tvc_completion_list, &vs->vs_completion_list);
 
 	vhost_work_queue(&vs->dev, &vs->vs_completion_work);
 }
@@ -347,27 +345,6 @@ static void vhost_scsi_free_cmd(struct tcm_vhost_cmd *tv_cmd)
 	kfree(tv_cmd);
 }
 
-/* Dequeue a command from the completion list */
-static struct tcm_vhost_cmd *vhost_scsi_get_cmd_from_completion(
-	struct vhost_scsi *vs)
-{
-	struct tcm_vhost_cmd *tv_cmd = NULL;
-
-	spin_lock_bh(&vs->vs_completion_lock);
-	if (list_empty(&vs->vs_completion_list)) {
-		spin_unlock_bh(&vs->vs_completion_lock);
-		return NULL;
-	}
-
-	list_for_each_entry(tv_cmd, &vs->vs_completion_list,
-			    tvc_completion_list) {
-		list_del(&tv_cmd->tvc_completion_list);
-		break;
-	}
-	spin_unlock_bh(&vs->vs_completion_lock);
-	return tv_cmd;
-}
-
 /* Fill in status and signal that we are done processing this command
  *
  * This is scheduled in the vhost work queue so we are called with the owner
@@ -377,13 +354,19 @@ static void vhost_scsi_complete_cmd_work(struct vhost_work *work)
 {
 	struct vhost_scsi *vs = container_of(work, struct vhost_scsi,
 					vs_completion_work);
-	struct tcm_vhost_cmd *tv_cmd;
-
-	while ((tv_cmd = vhost_scsi_get_cmd_from_completion(vs))) {
 	struct virtio_scsi_cmd_resp v_rsp;
-		struct se_cmd *se_cmd = &tv_cmd->tvc_se_cmd;
+	struct tcm_vhost_cmd *tv_cmd;
+	struct llist_node *llnode;
+	struct se_cmd *se_cmd;
 	int ret;
 
+	llnode = llist_del_all(&vs->vs_completion_list);
+	while (llnode) {
+		tv_cmd = llist_entry(llnode, struct tcm_vhost_cmd,
+				     tvc_completion_list);
+		llnode = llist_next(llnode);
+		se_cmd = &tv_cmd->tvc_se_cmd;
+
 		pr_debug("%s tv_cmd %p resid %u status %#02x\n", __func__,
 			tv_cmd, se_cmd->residual_count, se_cmd->scsi_status);
 
@@ -426,7 +409,6 @@ static struct tcm_vhost_cmd *vhost_scsi_allocate_cmd(
 		pr_err("Unable to allocate struct tcm_vhost_cmd\n");
 		return ERR_PTR(-ENOMEM);
 	}
-	INIT_LIST_HEAD(&tv_cmd->tvc_completion_list);
 	tv_cmd->tvc_tag = v_req->tag;
 	tv_cmd->tvc_task_attr = v_req->task_attr;
 	tv_cmd->tvc_exp_data_len = exp_data_len;
@@ -857,8 +839,6 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
 		return -ENOMEM;
 
 	vhost_work_init(&s->vs_completion_work, vhost_scsi_complete_cmd_work);
-	INIT_LIST_HEAD(&s->vs_completion_list);
-	spin_lock_init(&s->vs_completion_lock);
 
 	s->vqs[VHOST_SCSI_VQ_CTL].handle_kick = vhost_scsi_ctl_handle_kick;
 	s->vqs[VHOST_SCSI_VQ_EVT].handle_kick = vhost_scsi_evt_handle_kick;

drivers/vhost/tcm_vhost.h

@@ -34,7 +34,7 @@ struct tcm_vhost_cmd {
 	/* Sense buffer that will be mapped into outgoing status */
 	unsigned char tvc_sense_buf[TRANSPORT_SENSE_BUFFER];
 	/* Completed commands list, serviced from vhost worker thread */
-	struct list_head tvc_completion_list;
+	struct llist_node tvc_completion_list;
 };
 
 struct tcm_vhost_nexus {