sbappend*() and soreceive() of SB_MTXLOCK marked sockets uses `sb_mtx'
mutex(9) for protection, meanwhile buffer usage check and corresponding
sbwait() sleep still serialized by solock(). Mark udp(4) as SB_OWNLOCK
to avoid solock() serialisation and rely to `sb_mtx' mutex(9). The
`sb_state' and `sb_flags' modifications must be protected by `sb_mtx'
too. soo_ioctl() and  socantrcvmore() hunks were missed for unix(4)
sockets, but they are common for all sockets.

divert(4) and raw sockets also needs to be converted. I can include
conversion to this diff too.

sbwait() modification was tested with blocking recvfrom(2) test
programs.

Index: sys/kern/sys_socket.c
===================================================================
RCS file: /cvs/src/sys/kern/sys_socket.c,v
retrieving revision 1.62
diff -u -p -r1.62 sys_socket.c
--- sys/kern/sys_socket.c	26 Mar 2024 09:46:47 -0000	1.62
+++ sys/kern/sys_socket.c	29 Mar 2024 08:16:50 -0000
@@ -91,6 +91,7 @@ soo_ioctl(struct file *fp, u_long cmd, c
 
 	case FIOASYNC:
 		solock(so);
+		mtx_enter(&so->so_rcv.sb_mtx);
 		if (*(int *)data) {
 			so->so_rcv.sb_flags |= SB_ASYNC;
 			so->so_snd.sb_flags |= SB_ASYNC;
@@ -98,6 +99,7 @@ soo_ioctl(struct file *fp, u_long cmd, c
 			so->so_rcv.sb_flags &= ~SB_ASYNC;
 			so->so_snd.sb_flags &= ~SB_ASYNC;
 		}
+		mtx_leave(&so->so_rcv.sb_mtx);
 		sounlock(so);
 		break;
 
Index: sys/kern/uipc_socket.c
===================================================================
RCS file: /cvs/src/sys/kern/uipc_socket.c,v
retrieving revision 1.323
diff -u -p -r1.323 uipc_socket.c
--- sys/kern/uipc_socket.c	27 Mar 2024 22:47:53 -0000	1.323
+++ sys/kern/uipc_socket.c	29 Mar 2024 08:16:50 -0000
@@ -155,6 +155,8 @@ soalloc(const struct protosw *prp, int w
 	case AF_INET6:
 		switch (prp->pr_type) {
 		case SOCK_DGRAM:
+			so->so_rcv.sb_flags |= SB_OWNLOCK;
+			/* FALLTHROUGH */
 		case SOCK_RAW:
 			so->so_rcv.sb_flags |= SB_MTXLOCK;
 			break;
@@ -1392,7 +1394,9 @@ sosplice(struct socket *so, int fd, off_
 	 * we sleep, the socket buffers are not marked as spliced yet.
 	 */
 	if (somove(so, M_WAIT)) {
+		mtx_enter(&so->so_rcv.sb_mtx);
 		so->so_rcv.sb_flags |= SB_SPLICE;
+		mtx_leave(&so->so_rcv.sb_mtx);
 		sosp->so_snd.sb_flags |= SB_SPLICE;
 	}
 
@@ -1420,7 +1424,9 @@ sounsplice(struct socket *so, struct soc
 	task_del(sosplice_taskq, &so->so_splicetask);
 	timeout_del(&so->so_idleto);
 	sosp->so_snd.sb_flags &= ~SB_SPLICE;
+	mtx_enter(&so->so_rcv.sb_mtx);
 	so->so_rcv.sb_flags &= ~SB_SPLICE;
+	mtx_leave(&so->so_rcv.sb_mtx);
 	so->so_sp->ssp_socket = sosp->so_sp->ssp_soback = NULL;
 	/* Do not wakeup a socket that is about to be freed. */
 	if ((freeing & SOSP_FREEING_READ) == 0 && soreadable(so))
@@ -1678,6 +1684,7 @@ somove(struct socket *so, int wait)
 		pru_rcvd(so);
 
 	/* Receive buffer did shrink by len bytes, adjust oob. */
+	mtx_enter(&so->so_rcv.sb_mtx);
 	rcvstate = so->so_rcv.sb_state;
 	so->so_rcv.sb_state &= ~SS_RCVATMARK;
 	oobmark = so->so_oobmark;
@@ -1688,6 +1695,7 @@ somove(struct socket *so, int wait)
 		if (oobmark >= len)
 			oobmark = 0;
 	}
+	mtx_leave(&so->so_rcv.sb_mtx);
 
 	/*
 	 * Handle oob data.  If any malloc fails, ignore error.
Index: sys/kern/uipc_socket2.c
===================================================================
RCS file: /cvs/src/sys/kern/uipc_socket2.c,v
retrieving revision 1.146
diff -u -p -r1.146 uipc_socket2.c
--- sys/kern/uipc_socket2.c	27 Mar 2024 22:47:53 -0000	1.146
+++ sys/kern/uipc_socket2.c	29 Mar 2024 08:16:50 -0000
@@ -351,7 +351,9 @@ void
 socantrcvmore(struct socket *so)
 {
 	soassertlocked(so);
+	mtx_enter(&so->so_rcv.sb_mtx);
 	so->so_rcv.sb_state |= SS_CANTRCVMORE;
+	mtx_leave(&so->so_rcv.sb_mtx);
 	sorwakeup(so);
 }
 
Index: sys/nfs/nfs_socket.c
===================================================================
RCS file: /cvs/src/sys/nfs/nfs_socket.c,v
retrieving revision 1.146
diff -u -p -r1.146 nfs_socket.c
--- sys/nfs/nfs_socket.c	22 Mar 2024 07:15:04 -0000	1.146
+++ sys/nfs/nfs_socket.c	29 Mar 2024 08:16:50 -0000
@@ -371,7 +371,9 @@ nfs_connect(struct nfsmount *nmp, struct
 	error = soreserve(so, sndreserve, rcvreserve);
 	if (error)
 		goto bad_locked;
+	mtx_enter(&so->so_rcv.sb_mtx);
 	so->so_rcv.sb_flags |= SB_NOINTR;
+	mtx_leave(&so->so_rcv.sb_mtx);
 	so->so_snd.sb_flags |= SB_NOINTR;
 	sounlock(so);
 
Index: sys/nfs/nfs_syscalls.c
===================================================================
RCS file: /cvs/src/sys/nfs/nfs_syscalls.c,v
retrieving revision 1.122
diff -u -p -r1.122 nfs_syscalls.c
--- sys/nfs/nfs_syscalls.c	22 Mar 2024 07:15:04 -0000	1.122
+++ sys/nfs/nfs_syscalls.c	29 Mar 2024 08:16:51 -0000
@@ -297,12 +297,12 @@ nfssvc_addsock(struct file *fp, struct m
 		m_freem(m);
 	}
 	solock(so);
-	so->so_rcv.sb_flags &= ~SB_NOINTR;
 	mtx_enter(&so->so_rcv.sb_mtx);
+	so->so_rcv.sb_flags &= ~SB_NOINTR;
 	so->so_rcv.sb_timeo_nsecs = INFSLP;
 	mtx_leave(&so->so_rcv.sb_mtx);
-	so->so_snd.sb_flags &= ~SB_NOINTR;
 	mtx_enter(&so->so_snd.sb_mtx);
+	so->so_snd.sb_flags &= ~SB_NOINTR;
 	so->so_snd.sb_timeo_nsecs = INFSLP;
 	mtx_leave(&so->so_snd.sb_mtx);
 	sounlock(so);