diff --git a/sys/netinet/sctp_output.c b/sys/netinet/sctp_output.c
--- a/sys/netinet/sctp_output.c
+++ b/sys/netinet/sctp_output.c
@@ -6770,7 +6770,9 @@
 	/* now free everything */
 	if (ca->inp) {
 		/* Lets clear the flag to allow others to run. */
+		SCTP_INP_WLOCK(ca->inp);
 		ca->inp->sctp_flags &= ~SCTP_PCB_FLAGS_SND_ITERATOR_UP;
+		SCTP_INP_WUNLOCK(ca->inp);
 	}
 	sctp_m_freem(ca->m);
 	SCTP_FREE(ca, SCTP_M_COPYAL);
@@ -6825,10 +6827,6 @@
 	int ret;
 	struct sctp_copy_all *ca;
 
-	if (inp->sctp_flags & SCTP_PCB_FLAGS_SND_ITERATOR_UP) {
-		/* There is another. */
-		return (EBUSY);
-	}
 	if (uio->uio_resid > (ssize_t)SCTP_BASE_SYSCTL(sctp_sendall_limit)) {
 		/* You must not be larger than the limit! */
 		return (EMSGSIZE);
@@ -6846,6 +6844,18 @@
 	if (srcv) {
 		memcpy(&ca->sndrcv, srcv, sizeof(struct sctp_nonpad_sndrcvinfo));
 	}
+
+	/* Serialize. */
+	SCTP_INP_WLOCK(inp);
+	if ((inp->sctp_flags & SCTP_PCB_FLAGS_SND_ITERATOR_UP) != 0) {
+		SCTP_INP_WUNLOCK(inp);
+		sctp_m_freem(m);
+		SCTP_FREE(ca, SCTP_M_COPYAL);
+		return (EBUSY);
+	}
+	inp->sctp_flags |= SCTP_PCB_FLAGS_SND_ITERATOR_UP;
+	SCTP_INP_WUNLOCK(inp);
+
 	/*
 	 * take off the sendall flag, it would be bad if we failed to do
 	 * this :-0
@@ -6857,6 +6867,10 @@
 		ca->m = sctp_copy_out_all(uio, ca->sndlen);
 		if (ca->m == NULL) {
 			SCTP_FREE(ca, SCTP_M_COPYAL);
+			sctp_m_freem(m);
+			SCTP_INP_WLOCK(inp);
+			inp->sctp_flags &= ~SCTP_PCB_FLAGS_SND_ITERATOR_UP;
+			SCTP_INP_WUNLOCK(inp);
 			SCTP_LTRACE_ERR_RET(inp, NULL, NULL, SCTP_FROM_SCTP_OUTPUT, ENOMEM);
 			return (ENOMEM);
 		}
@@ -6869,14 +6883,15 @@
 			ca->sndlen += SCTP_BUF_LEN(mat);
 		}
 	}
-	inp->sctp_flags |= SCTP_PCB_FLAGS_SND_ITERATOR_UP;
 	ret = sctp_initiate_iterator(NULL, sctp_sendall_iterator, NULL,
 	    SCTP_PCB_ANY_FLAGS, SCTP_PCB_ANY_FEATURES,
 	    SCTP_ASOC_ANY_STATE,
 	    (void *)ca, 0,
 	    sctp_sendall_completes, inp, 1);
 	if (ret) {
+		SCTP_INP_WLOCK(inp);
 		inp->sctp_flags &= ~SCTP_PCB_FLAGS_SND_ITERATOR_UP;
+		SCTP_INP_WUNLOCK(inp);
 		SCTP_FREE(ca, SCTP_M_COPYAL);
 		SCTP_LTRACE_ERR_RET_PKT(m, inp, NULL, NULL, SCTP_FROM_SCTP_OUTPUT, EFAULT);
 		return (EFAULT);