scsi: virtio_scsi: fix IO hang caused by automatic irq vector affinity

Since commit 84676c1 ("genirq/affinity: assign vectors to all
possible CPUs") it is possible to end up in a scenario where only
offline CPUs are mapped to an interrupt vector.

This is only an issue for the legacy I/O path since with blk-mq/scsi-mq
an I/O can't be submitted to a hardware queue if the queue isn't mapped
to an online CPU.

Fix this issue by forcing virtio-scsi to use blk-mq.

[mkp: commit desc]

Cc: Omar Sandoval <osandov@fb.com>,
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>,
Cc: James Bottomley <james.bottomley@hansenpartnership.com>,
Cc: Christoph Hellwig <hch@lst.de>,
Cc: Don Brace <don.brace@microsemi.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Fixes: 84676c1 ("genirq/affinity: assign vectors to all possible CPUs")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/virtio_scsi.c

@@ -91,9 +91,6 @@ struct virtio_scsi_vq {
 struct virtio_scsi_target_state {
 	seqcount_t tgt_seq;
 
-	/* Count of outstanding requests. */
-	atomic_t reqs;
-
 	/* Currently active virtqueue for requests sent to this target. */
 	struct virtio_scsi_vq *req_vq;
 };
@@ -152,8 +149,6 @@ static void virtscsi_complete_cmd(struct virtio_scsi *vscsi, void *buf)
 	struct virtio_scsi_cmd *cmd = buf;
 	struct scsi_cmnd *sc = cmd->sc;
 	struct virtio_scsi_cmd_resp *resp = &cmd->resp.cmd;
-	struct virtio_scsi_target_state *tgt =
-				scsi_target(sc->device)->hostdata;
 
 	dev_dbg(&sc->device->sdev_gendev,
 		"cmd %p response %u status %#02x sense_len %u\n",
@@ -210,8 +205,6 @@ static void virtscsi_complete_cmd(struct virtio_scsi *vscsi, void *buf)
 	}
 
 	sc->scsi_done(sc);
-
-	atomic_dec(&tgt->reqs);
 }
 
 static void virtscsi_vq_done(struct virtio_scsi *vscsi,
@@ -580,10 +573,7 @@ static int virtscsi_queuecommand_single(struct Scsi_Host *sh,
 					struct scsi_cmnd *sc)
 {
 	struct virtio_scsi *vscsi = shost_priv(sh);
-	struct virtio_scsi_target_state *tgt =
-				scsi_target(sc->device)->hostdata;
 
-	atomic_inc(&tgt->reqs);
 	return virtscsi_queuecommand(vscsi, &vscsi->req_vqs[0], sc);
 }
 
@@ -596,55 +586,11 @@ static struct virtio_scsi_vq *virtscsi_pick_vq_mq(struct virtio_scsi *vscsi,
 	return &vscsi->req_vqs[hwq];
 }
 
-static struct virtio_scsi_vq *virtscsi_pick_vq(struct virtio_scsi *vscsi,
-					       struct virtio_scsi_target_state *tgt)
-{
-	struct virtio_scsi_vq *vq;
-	unsigned long flags;
-	u32 queue_num;
-
-	local_irq_save(flags);
-	if (atomic_inc_return(&tgt->reqs) > 1) {
-		unsigned long seq;
-
-		do {
-			seq = read_seqcount_begin(&tgt->tgt_seq);
-			vq = tgt->req_vq;
-		} while (read_seqcount_retry(&tgt->tgt_seq, seq));
-	} else {
-		/* no writes can be concurrent because of atomic_t */
-		write_seqcount_begin(&tgt->tgt_seq);
-
-		/* keep previous req_vq if a reader just arrived */
-		if (unlikely(atomic_read(&tgt->reqs) > 1)) {
-			vq = tgt->req_vq;
-			goto unlock;
-		}
-
-		queue_num = smp_processor_id();
-		while (unlikely(queue_num >= vscsi->num_queues))
-			queue_num -= vscsi->num_queues;
-		tgt->req_vq = vq = &vscsi->req_vqs[queue_num];
- unlock:
-		write_seqcount_end(&tgt->tgt_seq);
-	}
-	local_irq_restore(flags);
-
-	return vq;
-}
-
 static int virtscsi_queuecommand_multi(struct Scsi_Host *sh,
 				       struct scsi_cmnd *sc)
 {
 	struct virtio_scsi *vscsi = shost_priv(sh);
-	struct virtio_scsi_target_state *tgt =
-				scsi_target(sc->device)->hostdata;
-	struct virtio_scsi_vq *req_vq;
-
-	if (shost_use_blk_mq(sh))
-		req_vq = virtscsi_pick_vq_mq(vscsi, sc);
-	else
-		req_vq = virtscsi_pick_vq(vscsi, tgt);
+	struct virtio_scsi_vq *req_vq = virtscsi_pick_vq_mq(vscsi, sc);
 
 	return virtscsi_queuecommand(vscsi, req_vq, sc);
 }
@@ -775,7 +721,6 @@ static int virtscsi_target_alloc(struct scsi_target *starget)
 		return -ENOMEM;
 
 	seqcount_init(&tgt->tgt_seq);
-	atomic_set(&tgt->reqs, 0);
 	tgt->req_vq = &vscsi->req_vqs[0];
 
 	starget->hostdata = tgt;
@@ -823,6 +768,7 @@ static struct scsi_host_template virtscsi_host_template_single = {
 	.target_alloc = virtscsi_target_alloc,
 	.target_destroy = virtscsi_target_destroy,
 	.track_queue_depth = 1,
+	.force_blk_mq = 1,
 };
 
 static struct scsi_host_template virtscsi_host_template_multi = {
@@ -844,6 +790,7 @@ static struct scsi_host_template virtscsi_host_template_multi = {
 	.target_destroy = virtscsi_target_destroy,
 	.map_queues = virtscsi_map_queues,
 	.track_queue_depth = 1,
+	.force_blk_mq = 1,
 };
 
 #define virtscsi_config_get(vdev, fld) \