Index: sys/dev/proto/proto.h
===================================================================
--- sys/dev/proto/proto.h
+++ sys/dev/proto/proto.h
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2014, 2015 Marcel Moolenaar
+ * Copyright (c) 2014, 2015, 2019 Marcel Moolenaar
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -36,7 +36,8 @@
 #define	PROTO_RES_BUSDMA	11
 
 struct proto_res {
-	int		r_type;
+	int		r_type:8;
+	int		r_opened:1;
 	int		r_rid;
 	union {
 		struct resource *res;
@@ -47,13 +48,14 @@
 		void		*cookie;
 		struct cdev	*cdev;
 	} r_u;
-	uintptr_t	r_opened;
 };
 
 struct proto_softc {
 	device_t	sc_dev;
 	struct proto_res sc_res[PROTO_RES_MAX];
 	int		sc_rescnt;
+	int		sc_opencnt;
+	struct mtx	sc_mtx;
 };
 
 extern devclass_t proto_devclass;
Index: sys/dev/proto/proto_busdma.h
===================================================================
--- sys/dev/proto/proto_busdma.h
+++ sys/dev/proto/proto_busdma.h
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2015 Marcel Moolenaar
+ * Copyright (c) 2015, 2019 Marcel Moolenaar
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -60,6 +60,7 @@
 	LIST_HEAD(,proto_tag)	tags;
 	LIST_HEAD(,proto_md)	mds;
 	bus_dma_tag_t		bd_roottag;
+	struct mtx		mtx;
 };
 
 struct proto_busdma *proto_busdma_attach(struct proto_softc *);
Index: sys/dev/proto/proto_busdma.c
===================================================================
--- sys/dev/proto/proto_busdma.c
+++ sys/dev/proto/proto_busdma.c
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2015 Marcel Moolenaar
+ * Copyright (c) 2015, 2019 Marcel Moolenaar
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -36,6 +36,7 @@
 #include <sys/kernel.h>
 #include <sys/malloc.h>
 #include <sys/module.h>
+#include <sys/mutex.h>
 #include <sys/proc.h>
 #include <sys/queue.h>
 #include <sys/rman.h>
@@ -62,14 +63,15 @@
 
 static int
 proto_busdma_tag_create(struct proto_busdma *busdma, struct proto_tag *parent,
-    struct proto_ioc_busdma *ioc)
+    struct proto_ioc_busdma *ioc, struct proto_tag *tag)
 {
-	struct proto_tag *tag;
 
 	/* Make sure that when a boundary is specified, it's a power of 2 */
 	if (ioc->u.tag.bndry != 0 &&
-	    (ioc->u.tag.bndry & (ioc->u.tag.bndry - 1)) != 0)
+	    (ioc->u.tag.bndry & (ioc->u.tag.bndry - 1)) != 0) {
+		free(tag, M_PROTO_BUSDMA);
 		return (EINVAL);
+	}
 
 	/*
 	 * If nsegs is 1, ignore maxsegsz. What this means is that if we have
@@ -79,7 +81,6 @@
 	if (ioc->u.tag.maxsegsz > ioc->u.tag.maxsz || ioc->u.tag.nsegs == 1)
 		ioc->u.tag.maxsegsz = ioc->u.tag.maxsz;
 
-	tag = malloc(sizeof(*tag), M_PROTO_BUSDMA, M_WAITOK | M_ZERO);
 	if (parent != NULL) {
 		tag->parent = parent;
 		LIST_INSERT_HEAD(&parent->children, tag, peers);
@@ -172,13 +173,11 @@
 
 static int
 proto_busdma_mem_alloc(struct proto_busdma *busdma, struct proto_tag *tag,
-    struct proto_ioc_busdma *ioc)
+    struct proto_ioc_busdma *ioc, struct proto_md *md)
 {
 	struct proto_callback_bundle pcb;
-	struct proto_md *md;
 	int error;
 
-	md = malloc(sizeof(*md), M_PROTO_BUSDMA, M_WAITOK | M_ZERO);
 	md->tag = tag;
 
 	error = bus_dma_tag_create(busdma->bd_roottag, tag->align, tag->bndry,
@@ -227,12 +226,10 @@
 
 static int
 proto_busdma_md_create(struct proto_busdma *busdma, struct proto_tag *tag,
-    struct proto_ioc_busdma *ioc)
+    struct proto_ioc_busdma *ioc, struct proto_md *md)
 {
-	struct proto_md *md;
 	int error;
 
-	md = malloc(sizeof(*md), M_PROTO_BUSDMA, M_WAITOK | M_ZERO);
 	md->tag = tag;
 
 	error = bus_dma_tag_create(busdma->bd_roottag, tag->align, tag->bndry,
@@ -355,6 +352,7 @@
 	struct proto_busdma *busdma;
 
 	busdma = malloc(sizeof(*busdma), M_PROTO_BUSDMA, M_WAITOK | M_ZERO);
+	mtx_init(&busdma->mtx, "proto-busdma", NULL, MTX_DEF);
 	return (busdma);
 }
 
@@ -363,6 +361,7 @@
 {
 
 	proto_busdma_cleanup(sc, busdma);
+	mtx_destroy(&busdma->mtx);
 	free(busdma, M_PROTO_BUSDMA);
 	return (0);
 }
@@ -373,10 +372,12 @@
 	struct proto_md *md, *md1;
 	struct proto_tag *tag, *tag1;
 
+	mtx_lock(&busdma->mtx);
 	LIST_FOREACH_SAFE(md, &busdma->mds, mds, md1)
 		proto_busdma_md_destroy_internal(busdma, md);
 	LIST_FOREACH_SAFE(tag, &busdma->tags, tags, tag1)
 		proto_busdma_tag_destroy(busdma, tag);
+	mtx_unlock(&busdma->mtx);
 	return (0);
 }
 
@@ -384,23 +385,37 @@
 proto_busdma_ioctl(struct proto_softc *sc, struct proto_busdma *busdma,
     struct proto_ioc_busdma *ioc, struct thread *td)
 {
-	struct proto_tag *tag;
+	struct proto_tag *ptag, *tag;
 	struct proto_md *md;
 	int error;
 
+	/* Allocate structures prior to acquiring the lock */
+	switch (ioc->request) {
+	case PROTO_IOC_BUSDMA_TAG_CREATE:
+	case PROTO_IOC_BUSDMA_TAG_DERIVE:
+		tag = malloc(sizeof(*tag), M_PROTO_BUSDMA, M_WAITOK | M_ZERO);
+		break;
+	case PROTO_IOC_BUSDMA_MEM_ALLOC:
+	case PROTO_IOC_BUSDMA_MD_CREATE:
+		md = malloc(sizeof(*md), M_PROTO_BUSDMA, M_WAITOK | M_ZERO);
+		break;
+	}
+
+	mtx_lock(&busdma->mtx);
 	error = 0;
 	switch (ioc->request) {
 	case PROTO_IOC_BUSDMA_TAG_CREATE:
 		busdma->bd_roottag = bus_get_dma_tag(sc->sc_dev);
-		error = proto_busdma_tag_create(busdma, NULL, ioc);
+		error = proto_busdma_tag_create(busdma, NULL, ioc, tag);
 		break;
 	case PROTO_IOC_BUSDMA_TAG_DERIVE:
-		tag = proto_busdma_tag_lookup(busdma, ioc->key);
-		if (tag == NULL) {
+		ptag = proto_busdma_tag_lookup(busdma, ioc->key);
+		if (ptag == NULL) {
+			free(tag, M_PROTO_BUSDMA);
 			error = EINVAL;
 			break;
 		}
-		error = proto_busdma_tag_create(busdma, tag, ioc);
+		error = proto_busdma_tag_create(busdma, ptag, ioc, tag);
 		break;
 	case PROTO_IOC_BUSDMA_TAG_DESTROY:
 		tag = proto_busdma_tag_lookup(busdma, ioc->key);
@@ -413,10 +428,11 @@
 	case PROTO_IOC_BUSDMA_MEM_ALLOC:
 		tag = proto_busdma_tag_lookup(busdma, ioc->u.md.tag);
 		if (tag == NULL) {
+			free(md, M_PROTO_BUSDMA);
 			error = EINVAL;
 			break;
 		}
-		error = proto_busdma_mem_alloc(busdma, tag, ioc);
+		error = proto_busdma_mem_alloc(busdma, tag, ioc, md);
 		break;
 	case PROTO_IOC_BUSDMA_MEM_FREE:
 		md = proto_busdma_md_lookup(busdma, ioc->key);
@@ -429,10 +445,11 @@
 	case PROTO_IOC_BUSDMA_MD_CREATE:
 		tag = proto_busdma_tag_lookup(busdma, ioc->u.md.tag);
 		if (tag == NULL) {
+			free(md, M_PROTO_BUSDMA);
 			error = EINVAL;
 			break;
 		}
-		error = proto_busdma_md_create(busdma, tag, ioc);
+		error = proto_busdma_md_create(busdma, tag, ioc, md);
 		break;
 	case PROTO_IOC_BUSDMA_MD_DESTROY:
 		md = proto_busdma_md_lookup(busdma, ioc->key);
@@ -470,6 +487,7 @@
 		error = EINVAL;
 		break;
 	}
+	mtx_unlock(&busdma->mtx);
 	return (error);
 }
 
@@ -477,11 +495,17 @@
 proto_busdma_mmap_allowed(struct proto_busdma *busdma, vm_paddr_t physaddr)
 {
 	struct proto_md *md;
+	int result;
 
+	mtx_lock(&busdma->mtx);
+	result = 0;
 	LIST_FOREACH(md, &busdma->mds, mds) {
 		if (physaddr >= trunc_page(md->physaddr) &&
-		    physaddr <= trunc_page(md->physaddr + md->tag->maxsz))
-			return (1);
+		    physaddr <= trunc_page(md->physaddr + md->tag->maxsz)) {
+			result = 1;
+			break;
+		}
 	}
-	return (0);
+	mtx_unlock(&busdma->mtx);
+	return (result);
 }
Index: sys/dev/proto/proto_core.c
===================================================================
--- sys/dev/proto/proto_core.c
+++ sys/dev/proto/proto_core.c
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2014, 2015 Marcel Moolenaar
+ * Copyright (c) 2014, 2015, 2019 Marcel Moolenaar
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -184,6 +184,7 @@
 
 	sc = device_get_softc(dev);
 	sc->sc_dev = dev;
+	mtx_init(&sc->sc_mtx, "proto-softc", NULL, MTX_DEF);
 
 	for (res = 0; res < sc->sc_rescnt; res++) {
 		r = sc->sc_res + res;
@@ -231,15 +232,16 @@
 
 	sc = device_get_softc(dev);
 
-	/* Don't detach if we have open device files. */
-	for (res = 0; res < sc->sc_rescnt; res++) {
-		r = sc->sc_res + res;
-		if (r->r_opened)
-			return (EBUSY);
-	}
+	mtx_lock(&sc->sc_mtx);
+	if (sc->sc_opencnt == 0)
+		sc->sc_opencnt = -1;
+	mtx_unlock(&sc->sc_mtx);
+	if (sc->sc_opencnt > 0)
+		return (EBUSY);
 
 	for (res = 0; res < sc->sc_rescnt; res++) {
 		r = sc->sc_res + res;
+
 		switch (r->r_type) {
 		case SYS_RES_IRQ:
 			/* XXX TODO */
@@ -252,21 +254,25 @@
 			break;
 		case SYS_RES_MEMORY:
 		case SYS_RES_IOPORT:
+			destroy_dev(r->r_u.cdev);
 			bus_release_resource(dev, r->r_type, r->r_rid,
 			    r->r_d.res);
-			destroy_dev(r->r_u.cdev);
 			break;
 		case PROTO_RES_PCICFG:
 			destroy_dev(r->r_u.cdev);
 			break;
 		case PROTO_RES_BUSDMA:
-			proto_busdma_detach(sc, r->r_d.busdma);
 			destroy_dev(r->r_u.cdev);
+			proto_busdma_detach(sc, r->r_d.busdma);
 			break;
 		}
 		r->r_type = PROTO_RES_UNUSED;
 	}
+	mtx_lock(&sc->sc_mtx);
 	sc->sc_rescnt = 0;
+	sc->sc_opencnt = 0;
+	mtx_unlock(&sc->sc_mtx);
+	mtx_destroy(&sc->sc_mtx);
 	return (0);
 }
 
@@ -278,11 +284,23 @@
 proto_open(struct cdev *cdev, int oflags, int devtype, struct thread *td)
 {
 	struct proto_res *r;
+	struct proto_softc *sc;
+	int error;
 
-	r = cdev->si_drv2;
-	if (!atomic_cmpset_acq_ptr(&r->r_opened, 0UL, (uintptr_t)td->td_proc))
-		return (EBUSY);
-	return (0);
+	sc = cdev->si_drv1;
+	mtx_lock(&sc->sc_mtx);
+	if (sc->sc_opencnt >= 0) {
+		r = cdev->si_drv2;
+		if (!r->r_opened) {
+			r->r_opened = 1;
+			sc->sc_opencnt++;
+			error = 0;
+		} else
+			error = EBUSY;
+	} else
+		error = ENXIO;
+	mtx_unlock(&sc->sc_mtx);
+	return (error);
 }
 
 static int
@@ -290,14 +308,24 @@
 {
 	struct proto_res *r;
 	struct proto_softc *sc;
+	int error;
 
 	sc = cdev->si_drv1;
-	r = cdev->si_drv2;
-	if (!atomic_cmpset_acq_ptr(&r->r_opened, (uintptr_t)td->td_proc, 0UL))
-		return (ENXIO);
-	if (r->r_type == PROTO_RES_BUSDMA)
-		proto_busdma_cleanup(sc, r->r_d.busdma);
-	return (0);
+	mtx_lock(&sc->sc_mtx);
+	if (sc->sc_opencnt > 0) {
+		r = cdev->si_drv2;
+		if (r->r_opened) {
+			if (r->r_type == PROTO_RES_BUSDMA)
+				proto_busdma_cleanup(sc, r->r_d.busdma);
+			r->r_opened = 0;
+			sc->sc_opencnt--;
+			error = 0;
+		} else
+			error = ENXIO;
+	} else
+		error = ENXIO;
+	mtx_unlock(&sc->sc_mtx);
+	return (error);
 }
 
 static int