MFC r277101 (by imp):

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.

2 files changed, 20 insertions, 0 deletions

diff --git a/sys/cam/ata/ata_da.c b/sys/cam/ata/ata_da.c
index ed8eb4d..d924e17 100644
--- a/sys/cam/ata/ata_da.c
+++ b/sys/cam/ata/ata_da.c
@@ -1797,6 +1797,16 @@ adadone(struct cam_periph *periph, union ccb *done_ccb)
 
 			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);
diff --git a/sys/cam/scsi/scsi_da.c b/sys/cam/scsi/scsi_da.c
index 3266301..ac3093c 100644
--- a/sys/cam/scsi/scsi_da.c
+++ b/sys/cam/scsi/scsi_da.c
@@ -3048,6 +3048,16 @@ dadone(struct cam_periph *periph, union ccb *done_ccb)
 			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);