svn commit: r277101 - in head/sys/cam: ata scsi

[Warner Losh]

Author: imp
Date: Tue Jan 13 00:20:35 2015
New Revision: 277101
URL: https://svnweb.freebsd.org/changeset/base/277101

Log:
  Explain a bit of tricky code dealing with trims and how it prevents
  starvation. These side effects aren't obvious without extremely
  careful study, and are important to do just so.

Modified:
  head/sys/cam/ata/ata_da.c
  head/sys/cam/scsi/scsi_da.c

Modified: head/sys/cam/ata/ata_da.c
==============================================================================
--- head/sys/cam/ata/ata_da.c	Tue Jan 13 00:11:56 2015	(r277100)
+++ head/sys/cam/ata/ata_da.c	Tue Jan 13 00:20:35 2015	(r277101)
@@ -1790,6 +1790,16 @@ adadone(struct cam_periph *periph, union
 
 			TAILQ_INIT(&queue);
 			TAILQ_CONCAT(&queue, &softc->trim_req.bps, bio_queue);
+			/*
+			 * Normally, the xpt_release_ccb() above would make sure
+			 * that when we have more work to do, that work would
+			 * get kicked off. However, we specifically keep
+			 * trim_running set to 0 before the call above to allow
+			 * other I/O to progress when many BIO_DELETE requests
+			 * are pushed down. We set trim_running to 0 and call
+			 * daschedule again so that we don't stall if there are
+			 * no other I/Os pending apart from BIO_DELETEs.
+			 */
 			softc->trim_running = 0;
 			adaschedule(periph);
 			cam_periph_unlock(periph);

Modified: head/sys/cam/scsi/scsi_da.c
==============================================================================
--- head/sys/cam/scsi/scsi_da.c	Tue Jan 13 00:11:56 2015	(r277100)
+++ head/sys/cam/scsi/scsi_da.c	Tue Jan 13 00:20:35 2015	(r277101)
@@ -3018,6 +3018,16 @@ dadone(struct cam_periph *periph, union 
 			TAILQ_INIT(&queue);
 			TAILQ_CONCAT(&queue, &softc->delete_run_queue.queue, bio_queue);
 			softc->delete_run_queue.insert_point = NULL;
+			/*
+			 * Normally, the xpt_release_ccb() above would make sure
+			 * that when we have more work to do, that work would
+			 * get kicked off. However, we specifically keep
+			 * delete_running set to 0 before the call above to
+			 * allow other I/O to progress when many BIO_DELETE
+			 * requests are pushed down. We set delete_running to 0
+			 * and call daschedule again so that we don't stall if
+			 * there are no other I/Os pending apart from BIO_DELETEs.
+			 */
 			softc->delete_running = 0;
 			daschedule(periph);
 			cam_periph_unlock(periph);