MFC r277440 (by will):

Restore the CAM XPT peripheral generation counter, and export it via sysctl.

Define it as an atomic uint32_t.  These increments happen infrequently
enough for the atomic overhead to be a problem, and since they're now
independent atomics, they won't contend with xpt_lock_buses().

This counter is useful as a means of cheaply identifying whether any changes
have been made to the CAM peripheral list.  Userland programs have no guarantee
that the counter won't change on them while being returned or while processing
the information, so they must be written accordingly.

1 files changed, 6 insertions, 0 deletions

diff --git a/sys/cam/cam_xpt.c b/sys/cam/cam_xpt.c
index 645ded0..17b4f19 100644
--- a/sys/cam/cam_xpt.c
+++ b/sys/cam/cam_xpt.c
@@ -93,6 +93,8 @@ struct xpt_task {
 };
 
 struct xpt_softc {
+	uint32_t		xpt_generation;
+
 	/* number of high powered commands that can go through right now */
 	struct mtx		xpt_highpower_lock;
 	STAILQ_HEAD(highpowerlist, cam_ed)	highpowerq;
@@ -154,6 +156,8 @@ MTX_SYSINIT(xpt_topo_init, &xsoftc.xpt_topo_lock, "XPT topology lock", MTX_DEF);
 TUNABLE_INT("kern.cam.boot_delay", &xsoftc.boot_delay);
 SYSCTL_INT(_kern_cam, OID_AUTO, boot_delay, CTLFLAG_RDTUN,
            &xsoftc.boot_delay, 0, "Bus registration wait time");
+SYSCTL_UINT(_kern_cam, OID_AUTO, xpt_generation, CTLFLAG_RD,
+	    &xsoftc.xpt_generation, 0, "CAM peripheral generation count");
 
 struct cam_doneq {
 	struct mtx_padalign	cam_doneq_mtx;
@@ -981,6 +985,7 @@ xpt_add_periph(struct cam_periph *periph)
 		device->generation++;
 		SLIST_INSERT_HEAD(&device->periphs, periph, periph_links);
 		mtx_unlock(&device->target->bus->eb_mtx);
+		atomic_add_32(&xsoftc.xpt_generation, 1);
 	}
 
 	return (status);
@@ -997,6 +1002,7 @@ xpt_remove_periph(struct cam_periph *periph)
 		device->generation++;
 		SLIST_REMOVE(&device->periphs, periph, cam_periph, periph_links);
 		mtx_unlock(&device->target->bus->eb_mtx);
+		atomic_add_32(&xsoftc.xpt_generation, 1);
 	}
 }