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>

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 6f10afaca25b2eb03c4088b28f1d53221dc62d88..22a7e4c47207a722b3a1b554cbeaafd7b68fc39a 100644 (file)
--- 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) {