svn commit: r340024 - in head/sys/cam: . scsi

[Warner Losh]

Author: imp
Date: Thu Nov  1 21:51:41 2018
New Revision: 340024
URL: https://svnweb.freebsd.org/changeset/base/340024

Log:
  Add comments explaining what hold/unhold do
  
  They act as a simple one-deep semaphore to keep open/close/probe from
  running at the same time to avoid races that creates.

Modified:
  head/sys/cam/cam_periph.c
  head/sys/cam/scsi/scsi_da.c

Modified: head/sys/cam/cam_periph.c
==============================================================================
--- head/sys/cam/cam_periph.c	Thu Nov  1 21:49:49 2018	(r340023)
+++ head/sys/cam/cam_periph.c	Thu Nov  1 21:51:41 2018	(r340024)
@@ -470,6 +470,12 @@ cam_periph_release(struct cam_periph *periph)
 	mtx_unlock(mtx);
 }
 
+/*
+ * hold/unhold act as mutual exclusion for sections of the code that
+ * need to sleep and want to make sure that other sections that
+ * will interfere are held off. This only protects exclusive sections
+ * from each other.
+ */
 int
 cam_periph_hold(struct cam_periph *periph, int priority)
 {

Modified: head/sys/cam/scsi/scsi_da.c
==============================================================================
--- head/sys/cam/scsi/scsi_da.c	Thu Nov  1 21:49:49 2018	(r340023)
+++ head/sys/cam/scsi/scsi_da.c	Thu Nov  1 21:51:41 2018	(r340024)
@@ -2697,9 +2697,10 @@ daregister(struct cam_periph *periph, void *arg)
 	TASK_INIT(&softc->sysctl_task, 0, dasysctlinit, periph);
 
 	/*
-	 * Take an exclusive refcount on the periph while dastart is called
-	 * to finish the probe.  The reference will be dropped in dadone at
-	 * the end of probe.
+	 * Take an exclusive section lock qon the periph while dastart is called
+	 * to finish the probe.  The lock will be dropped in dadone at the end
+	 * of probe. This locks out daopen and daclose from racing with the
+	 * probe.
 	 *
 	 * XXX if cam_periph_hold returns an error, we don't hold a refcount.
 	 */