scsi: sd_zbc: Avoid that resetting a zone fails sporadically

Since SCSI scanning occurs asynchronously, since sd_revalidate_disk() is
called from sd_probe_async() and since sd_revalidate_disk() calls
sd_zbc_read_zones() it can happen that sd_zbc_read_zones() is called
concurrently with blkdev_report_zones() and/or blkdev_reset_zones().  That can
cause these functions to fail with -EIO because sd_zbc_read_zones() e.g. sets
q->nr_zones to zero before restoring it to the actual value, even if no drive
characteristics have changed.  Avoid that this can happen by making the
following changes:

- Protect the code that updates zone information with blk_queue_enter()
  and blk_queue_exit().
- Modify sd_zbc_setup_seq_zones_bitmap() and sd_zbc_setup() such that
  these functions do not modify struct scsi_disk before all zone
  information has been obtained.

Note: since commit 055f6e18e0 ("block: Make q_usage_counter also track
legacy requests"; kernel v4.15) the request queue freezing mechanism also
affects legacy request queues.

Fixes: 89d9475610 ("sd: Implement support for ZBC devices")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: stable@vger.kernel.org # v4.16
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/linux/blkdev.h

@@ -605,6 +605,11 @@ struct request_queue {
 	 * initialized by the low level device driver (e.g. scsi/sd.c).
 	 * Stacking drivers (device mappers) may or may not initialize
 	 * these fields.
+	 *
+	 * Reads of this information must be protected with blk_queue_enter() /
+	 * blk_queue_exit(). Modifying this information is only allowed while
+	 * no requests are being processed. See also blk_mq_freeze_queue() and
+	 * blk_mq_unfreeze_queue().
 	 */
 	unsigned int		nr_zones;
 	unsigned long		*seq_zones_bitmap;