SCSI: don't get target/host busy_count in scsi_mq_get_budget()

It is very expensive to atomic_inc/atomic_dec the host wide counter of
host->busy_count, and it should have been avoided via blk-mq's mechanism
of getting driver tag, which uses the more efficient way of sbitmap queue.

Also we don't check atomic_read(&sdev->device_busy) in scsi_mq_get_budget()
and don't run queue if the counter becomes zero, so IO hang may be caused
if all requests are completed just before the current SCSI device
is added to shost->starved_list.

Fixes: 0df21c86bdbf(scsi: implement .get_budget and .put_budget for blk-mq)
Reported-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 13 insertions, 16 deletions

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 6f10afaca25b..22a7e4c47207 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1950,11 +1950,7 @@ static void scsi_mq_put_budget(struct blk_mq_hw_ctx *hctx)
 {
         struct request_queue *q = hctx->queue;
         struct scsi_device *sdev = q->queuedata;
-        struct Scsi_Host *shost = sdev->host;
 
-        atomic_dec(&shost->host_busy);
-        if (scsi_target(sdev)->can_queue > 0)
-                atomic_dec(&scsi_target(sdev)->target_busy);
         atomic_dec(&sdev->device_busy);
         put_device(&sdev->sdev_gendev);
 }
@@ -1963,7 +1959,6 @@ static blk_status_t scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
 {
         struct request_queue *q = hctx->queue;
         struct scsi_device *sdev = q->queuedata;
-        struct Scsi_Host *shost = sdev->host;
         blk_status_t ret;
 
         ret = prep_to_mq(scsi_prep_state_check(sdev, NULL));
@@ -1974,18 +1969,9 @@ static blk_status_t scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
                 goto out;
         if (!scsi_dev_queue_ready(q, sdev))
                 goto out_put_device;
-        if (!scsi_target_queue_ready(shost, sdev))
-                goto out_dec_device_busy;
-        if (!scsi_host_queue_ready(q, shost, sdev))
-                goto out_dec_target_busy;
 
         return BLK_STS_OK;
 
-out_dec_target_busy:
-        if (scsi_target(sdev)->can_queue > 0)
-                atomic_dec(&scsi_target(sdev)->target_busy);
-out_dec_device_busy:
-        atomic_dec(&sdev->device_busy);
 out_put_device:
         put_device(&sdev->sdev_gendev);
 out:
@@ -1998,6 +1984,7 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
         struct request *req = bd->rq;
         struct request_queue *q = req->q;
         struct scsi_device *sdev = q->queuedata;
+        struct Scsi_Host *shost = sdev->host;
         struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);
         blk_status_t ret;
         int reason;
@@ -2007,10 +1994,15 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
                 goto out_put_budget;
 
         ret = BLK_STS_RESOURCE;
+        if (!scsi_target_queue_ready(shost, sdev))
+                goto out_put_budget;
+        if (!scsi_host_queue_ready(q, shost, sdev))
+                goto out_dec_target_busy;
+
         if (!(req->rq_flags & RQF_DONTPREP)) {
                 ret = prep_to_mq(scsi_mq_prep_fn(req));
                 if (ret != BLK_STS_OK)
-                        goto out_put_budget;
+                        goto out_dec_host_busy;
                 req->rq_flags |= RQF_DONTPREP;
         } else {
                 blk_mq_start_request(req);
@@ -2028,11 +2020,16 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
         if (reason) {
                 scsi_set_blocked(cmd, reason);
                 ret = BLK_STS_RESOURCE;
-                goto out_put_budget;
+                goto out_dec_host_busy;
         }
 
         return BLK_STS_OK;
 
+out_dec_host_busy:
+       atomic_dec(&shost->host_busy);
+out_dec_target_busy:
+        if (scsi_target(sdev)->can_queue > 0)
+                atomic_dec(&scsi_target(sdev)->target_busy);
 out_put_budget:
         scsi_mq_put_budget(hctx);
         switch (ret) {