Index: sys/fs/devfs/devfs_devs.c
===================================================================
--- sys/fs/devfs/devfs_devs.c
+++ sys/fs/devfs/devfs_devs.c
@@ -144,6 +144,7 @@
 	LIST_INIT(&cdev->si_children);
 	vfs_timestamp(&ts);
 	cdev->si_atime = cdev->si_mtime = cdev->si_ctime = ts;
+	mtx_init(&cdev->si_threadlock, "devthrd", NULL, MTX_DEF);
 
 	return (cdev);
 }
@@ -180,6 +181,7 @@
 	devfs_free_cdp_inode(cdp->cdp_inode);
 	if (cdp->cdp_maxdirent > 0) 
 		free(cdp->cdp_dirents, M_DEVFS2);
+	mtx_destroy(&cdev->si_threadlock);
 	free(cdp, M_CDEVP);
 }
 
Index: sys/kern/kern_conf.c
===================================================================
--- sys/kern/kern_conf.c
+++ sys/kern/kern_conf.c
@@ -186,7 +186,7 @@
 		*ref = 0;
 		return (dev->si_devsw);
 	}
-	dev_lock();
+	mtx_lock(&dev->si_threadlock);
 	csw = dev->si_devsw;
 	if (csw != NULL) {
 		cdp = cdev2priv(dev);
@@ -195,7 +195,7 @@
 		else
 			csw = NULL;
 	}
-	dev_unlock();
+	mtx_unlock(&dev->si_threadlock);
 	if (csw != NULL)
 		*ref = 1;
 	return (csw);
@@ -223,19 +223,21 @@
 	}
 
 	csw = NULL;
-	dev_lock();
+	VI_LOCK(vp);
 	dev = vp->v_rdev;
 	if (dev == NULL) {
-		dev_unlock();
+		VI_UNLOCK(vp);
 		return (NULL);
 	}
+	mtx_lock(&dev->si_threadlock);
 	cdp = cdev2priv(dev);
 	if ((cdp->cdp_flags & CDP_SCHED_DTR) == 0) {
 		csw = dev->si_devsw;
 		if (csw != NULL)
 			atomic_add_long(&dev->si_threadcount, 1);
 	}
-	dev_unlock();
+	mtx_unlock(&dev->si_threadlock);
+	VI_UNLOCK(vp);
 	if (csw != NULL) {
 		*devp = dev;
 		*ref = 1;
@@ -1133,20 +1135,26 @@
 		dev->si_flags &= ~SI_CLONELIST;
 	}
 
+	mtx_lock(&dev->si_threadlock);
 	csw = dev->si_devsw;
 	dev->si_devsw = NULL;	/* already NULL for SI_ALIAS */
 	while (csw != NULL && csw->d_purge != NULL && dev->si_threadcount) {
 		csw->d_purge(dev);
+		mtx_unlock(&dev->si_threadlock);
 		msleep(csw, &devmtx, PRIBIO, "devprg", hz/10);
+		mtx_lock(&dev->si_threadlock);
 		if (dev->si_threadcount)
 			printf("Still %lu threads in %s\n",
 			    dev->si_threadcount, devtoname(dev));
 	}
 	while (dev->si_threadcount != 0) {
 		/* Use unique dummy wait ident */
+		mtx_unlock(&dev->si_threadlock);
 		msleep(&csw, &devmtx, PRIBIO, "devdrn", hz / 10);
+		mtx_lock(&dev->si_threadlock);
 	}
 
+	mtx_unlock(&dev->si_threadlock);
 	dev_unlock();
 	if ((cdp->cdp_flags & CDP_UNREF_DTR) == 0) {
 		/* avoid out of order notify events */
Index: sys/kern/subr_witness.c
===================================================================
--- sys/kern/subr_witness.c
+++ sys/kern/subr_witness.c
@@ -601,6 +601,7 @@
 	{ "vm map (system)", &lock_class_mtx_sleep },
 	{ "vnode interlock", &lock_class_mtx_sleep },
 	{ "cdev", &lock_class_mtx_sleep },
+	{ "devthrd", &lock_class_mtx_sleep },
 	{ NULL, NULL },
 	/*
 	 * VM
Index: sys/sys/conf.h
===================================================================
--- sys/sys/conf.h
+++ sys/sys/conf.h
@@ -48,6 +48,9 @@
 #include <sys/queue.h>
 #endif
 
+#include <sys/_lock.h>
+#include <sys/_mutex.h>
+
 struct snapdata;
 struct devfs_dirent;
 struct cdevsw;
@@ -84,6 +87,7 @@
 	struct cdevsw	*si_devsw;
 	int		si_iosize_max;	/* maximum I/O size (for physio &al) */
 	u_long		si_usecount;
+	struct mtx	si_threadlock; /* protects si_devsw association */
 	u_long		si_threadcount;
 	union {
 		struct snapdata *__sid_snapdata;
@@ -167,7 +171,8 @@
 #define	D_VERSION_02	0x28042009	/* Add d_mmap_single */
 #define	D_VERSION_03	0x17122009	/* d_mmap takes memattr,vm_ooffset_t */
 #define	D_VERSION_04	0x5c48c353	/* SPECNAMELEN bumped to MAXNAMLEN */
-#define	D_VERSION	D_VERSION_04
+#define	D_VERSION_05	0x5ddc0b6c	/* per-dev lock to protect si_devsw */
+#define	D_VERSION	D_VERSION_05
 
 /*
  * Flags used for internal housekeeping