diff --git a/sys/compat/freebsd32/syscalls.master b/sys/compat/freebsd32/syscalls.master
index 1afe586a2b08..4f1fc28337d4 100644
--- a/sys/compat/freebsd32/syscalls.master
+++ b/sys/compat/freebsd32/syscalls.master
@@ -386,16 +386,16 @@
 ;
 ; The following are reserved for loadable syscalls
 ;
-210	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-211	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-212	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-213	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-214	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-215	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-216	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-217	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-218	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-219	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
+210	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+211	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+212	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+213	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+214	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+215	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+216	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+217	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+218	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+219	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
 
 ;
 ; The following were introduced with NetBSD/4.4Lite-2
diff --git a/sys/kern/kern_syscalls.c b/sys/kern/kern_syscalls.c
index 9cb7b68bd541..d1a5c0dba5f7 100644
--- a/sys/kern/kern_syscalls.c
+++ b/sys/kern/kern_syscalls.c
@@ -28,12 +28,15 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
+#include <sys/kernel.h>
 #include <sys/lock.h>
 #include <sys/module.h>
 #include <sys/sx.h>
 #include <sys/syscall.h>
 #include <sys/sysent.h>
 #include <sys/sysproto.h>
+#include <sys/systm.h>
+#include <machine/atomic.h>
 
 /*
  * Acts like "nosys" but can be identified in sysent for dynamic call
@@ -55,6 +58,51 @@ lkmressys(struct thread *td, struct nosys_args *args)
 	return (nosys(td, args));
 }
 
+static void
+syscall_thread_drain(struct sysent *se)
+{
+	u_int32_t cnt, oldcnt;
+
+	do {
+		oldcnt = se->sy_thrcnt;
+		KASSERT((oldcnt & SY_THR_STATIC) == 0,
+		    ("drain on static syscall"));
+		cnt = oldcnt | SY_THR_DRAINING;
+	} while (atomic_cmpset_acq_32(&se->sy_thrcnt, oldcnt, cnt) == 0);
+	while (atomic_cmpset_32(&se->sy_thrcnt, SY_THR_DRAINING,
+	    SY_THR_ABSENT) == 0)
+		pause("scdrn", hz/2);
+}
+
+int
+syscall_thread_enter(struct thread *td, struct sysent *se)
+{
+	u_int32_t cnt, oldcnt;
+
+	do {
+		oldcnt = se->sy_thrcnt;
+		if ((oldcnt & SY_THR_STATIC) != 0)
+			return (0);
+		if ((oldcnt & (SY_THR_DRAINING | SY_THR_ABSENT)) != 0)
+			return (ENOSYS);
+		cnt = oldcnt + SY_THR_INCR;
+	} while (atomic_cmpset_acq_32(&se->sy_thrcnt, oldcnt, cnt) == 0);
+	return (0);
+}
+
+void
+syscall_thread_exit(struct thread *td, struct sysent *se)
+{
+	u_int32_t cnt, oldcnt;
+
+	do {
+		oldcnt = se->sy_thrcnt;
+		if ((oldcnt & SY_THR_STATIC) != 0)
+			return;
+		cnt = oldcnt - SY_THR_INCR;
+	} while (atomic_cmpset_rel_32(&se->sy_thrcnt, oldcnt, cnt) == 0);
+}
+
 int
 syscall_register(int *offset, struct sysent *new_sysent,
     struct sysent *old_sysent)
@@ -74,8 +122,12 @@ syscall_register(int *offset, struct sysent *new_sysent,
 	    sysent[*offset].sy_call != (sy_call_t *)lkmressys)
 		return (EEXIST);
 
+	KASSERT(sysent[*offset].sy_thrcnt == SY_THR_ABSENT,
+	    ("dynamic syscall is not protected"));
 	*old_sysent = sysent[*offset];
+	new_sysent->sy_thrcnt = SY_THR_ABSENT;
 	sysent[*offset] = *new_sysent;
+	atomic_store_rel_32(&sysent[*offset].sy_thrcnt, 0);
 	return (0);
 }
 
@@ -83,8 +135,10 @@ int
 syscall_deregister(int *offset, struct sysent *old_sysent)
 {
 
-	if (*offset)
+	if (*offset) {
+		syscall_thread_drain(&sysent[*offset]);
 		sysent[*offset] = *old_sysent;
+	}
 	return (0);
 }
 
diff --git a/sys/kern/makesyscalls.sh b/sys/kern/makesyscalls.sh
index 4e55f96857cc..46e04fc53029 100644
--- a/sys/kern/makesyscalls.sh
+++ b/sys/kern/makesyscalls.sh
@@ -253,6 +253,10 @@ s/\$//g
 		f=4			# toss number, type, audit event
 		argc= 0;
 		argssize = "0"
+		thr_flag = "SY_THR_STATIC"
+		if (flag("NOTSTATIC")) {
+			thr_flag = "SY_THR_ABSENT"
+		}
 		if ($NF != "}") {
 			funcalias=$(NF-2)
 			argalias=$(NF-1)
@@ -401,10 +405,10 @@ s/\$//g
 		printf("\t{ %s, (sy_call_t *)", argssize) > sysent
 		column = 8 + 2 + length(argssize) + 15
 		if (flag("NOSTD")) {
-			printf("%s },", "lkmressys, AUE_NULL, NULL, 0, 0, 0") > sysent
+			printf("%s },", "lkmressys, AUE_NULL, NULL, 0, 0, 0, SY_THR_ABSENT") > sysent
 			column = column + length("lkmressys") + length("AUE_NULL") + 3
 		} else {
-			printf("%s, %s, NULL, 0, 0, %s },", funcname, auditev, flags) > sysent
+			printf("%s, %s, NULL, 0, 0, %s, %s },", funcname, auditev, flags, thr_flag) > sysent
 			column = column + length(funcname) + length(auditev) + length(flags) + 3
 		} 
 		align_sysent_comment(column)
@@ -472,13 +476,13 @@ s/\$//g
 			    prefix, funcname, auditev) > sysaue
 		}
 		if (flag("NOSTD")) {
-			printf("\t{ %s, (sy_call_t *)%s, %s, NULL, 0, 0, 0 },",
+			printf("\t{ %s, (sy_call_t *)%s, %s, NULL, 0, 0, 0, SY_THR_ABSENT },",
 			    "0", "lkmressys", "AUE_NULL") > sysent
 			align_sysent_comment(8 + 2 + length("0") + 15 + \
 			    length("lkmressys") + length("AUE_NULL") + 3)
 		} else {
-			printf("\t{ %s(%s,%s), %s, NULL, 0, 0, %s },",
-			    wrap, argssize, funcname, auditev, flags) > sysent
+			printf("\t{ %s(%s,%s), %s, NULL, 0, 0, %s, %s },",
+			    wrap, argssize, funcname, auditev, flags, thr_flag) > sysent
 			align_sysent_comment(8 + 9 + length(argssize) + 1 + \
 			    length(funcname) + length(auditev) + \
 			    length(flags) + 4)
@@ -498,7 +502,7 @@ s/\$//g
 		next
 	}
 	type("OBSOL") {
-		printf("\t{ 0, (sy_call_t *)nosys, AUE_NULL, NULL, 0, 0, 0 },") > sysent
+		printf("\t{ 0, (sy_call_t *)nosys, AUE_NULL, NULL, 0, 0, 0, SY_THR_ABSENT },") > sysent
 		align_sysent_comment(34)
 		printf("/* %d = obsolete %s */\n", syscall, comment) > sysent
 		printf("\t\"obs_%s\",\t\t\t/* %d = obsolete %s */\n",
@@ -509,7 +513,7 @@ s/\$//g
 		next
 	}
 	type("UNIMPL") {
-		printf("\t{ 0, (sy_call_t *)nosys, AUE_NULL, NULL, 0, 0, 0 },\t\t\t/* %d = %s */\n",
+		printf("\t{ 0, (sy_call_t *)nosys, AUE_NULL, NULL, 0, 0, 0, SY_THR_ABSENT },\t\t\t/* %d = %s */\n",
 		    syscall, comment) > sysent
 		printf("\t\"#%d\",\t\t\t/* %d = %s */\n",
 		    syscall, syscall, comment) > sysnames
diff --git a/sys/kern/subr_trap.c b/sys/kern/subr_trap.c
index 19bd5d46684d..4298429223bf 100644
--- a/sys/kern/subr_trap.c
+++ b/sys/kern/subr_trap.c
@@ -298,6 +298,9 @@ syscallenter(struct thread *td, struct syscall_args *sa)
 			if (error != 0)
 				goto retval;
 		}
+		error = syscall_thread_enter(td, sa->callp);
+		if (error != 0)
+			goto retval;
 
 #ifdef KDTRACE_HOOKS
 		/*
@@ -327,6 +330,7 @@ syscallenter(struct thread *td, struct syscall_args *sa)
 			(*systrace_probe_func)(sa->callp->sy_return, sa->code,
 			    sa->callp, sa->args);
 #endif
+		syscall_thread_exit(td, sa->callp);
 		CTR4(KTR_SYSC, "syscall: p=%p error=%d return %#lx %#lx",
 		    p, error, td->td_retval[0], td->td_retval[1]);
 	}
diff --git a/sys/kern/syscalls.master b/sys/kern/syscalls.master
index 6a0ea65f7d53..8e11134d06e9 100644
--- a/sys/kern/syscalls.master
+++ b/sys/kern/syscalls.master
@@ -40,7 +40,7 @@
 ;	NOPROTO	same as STD except do not create structure or
 ;		function prototype in sys/sysproto.h.  Does add a
 ;		definition to syscall.h besides adding a sysent.
-
+;	NONSTATIC syscall is loadable
 ;
 ; Please copy any additions and changes to the following compatability tables:
 ; sys/compat/freebsd32/syscalls.master
@@ -403,16 +403,16 @@
 ;
 ; The following are reserved for loadable syscalls
 ;
-210	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-211	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-212	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-213	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-214	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-215	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-216	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-217	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-218	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
-219	AUE_NULL	NODEF	lkmnosys lkmnosys nosys_args int
+210	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+211	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+212	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+213	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+214	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+215	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+216	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+217	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+218	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
+219	AUE_NULL	NODEF|NOTSTATIC	lkmnosys lkmnosys nosys_args int
 
 ;
 ; The following were introduced with NetBSD/4.4Lite-2
diff --git a/sys/sys/sysent.h b/sys/sys/sysent.h
index 99e44a0d44ab..641bdae58d49 100644
--- a/sys/sys/sysent.h
+++ b/sys/sys/sysent.h
@@ -61,8 +61,15 @@ struct sysent {			/* system call table */
 	u_int32_t sy_entry;	/* DTrace entry ID for systrace. */
 	u_int32_t sy_return;	/* DTrace return ID for systrace. */
 	u_int32_t sy_flags;	/* General flags for system calls. */
+	u_int32_t sy_thrcnt;
 };
 
+#define	SY_THR_FLAGMASK	0x7
+#define	SY_THR_STATIC	0x1
+#define	SY_THR_DRAINING	0x2
+#define	SY_THR_ABSENT	0x4
+#define	SY_THR_INCR	0x8
+
 struct image_params;
 struct __sigset;
 struct syscall_args;
@@ -211,6 +218,9 @@ struct nosys_args;
 int	lkmnosys(struct thread *, struct nosys_args *);
 int	lkmressys(struct thread *, struct nosys_args *);
 
+int	syscall_thread_enter(struct thread *td, struct sysent *se);
+void	syscall_thread_exit(struct thread *td, struct sysent *se);
+
 #endif /* _KERNEL */
 
 #endif /* !_SYS_SYSENT_H_ */