Replace 4.4BSD Lite's unix domain socket backpressure hack with a cleaner

mechanism, based on the new SB_STOP sockbuf flag.  The old hack dynamically
changed the sending sockbuf's high water mark whenever adding or removing
data from the receiving sockbuf.  It worked for stream sockets, but it never
worked for SOCK_SEQPACKET sockets because of their atomic nature.  If the
sockbuf was partially full, it might return EMSGSIZE instead of blocking.

The new solution is based on DragonFlyBSD's fix from commit
3a6117bbe0ed6a87605c1e43e12a1438d8844380 on 2008-05-27.  It adds an SB_STOP
flag to sockbufs.  Whenever uipc_send surpasses the socket's size limit, it
sets SB_STOP on the sending sockbuf.  sbspace() will then return 0 for that
sockbuf, causing sosend_generic and friends to block.  uipc_rcvd will
likewise clear SB_STOP.  There are two fringe benefits: uipc_{send,rcvd} no
longer need to call chgsbsize() on every send and receive because they don't
change the sockbuf's high water mark.  Also, uipc_sense no longer needs to
acquire the UIPC linkage lock, because it's simpler to compute the
st_blksizes.

There is one drawback: since sbspace() will only ever return 0 or the
maximum, sosend_generic will allow the sockbuf to exceed its nominal maximum
size by at most one packet of size less than the max.  I don't think that's
a serious problem.  In fact, I'm not even positive that FreeBSD guarantees a
socket will always stay within its nominal size limit.

sys/sys/sockbuf.h
	Add the SB_STOP flag and adjust sbspace()

sys/sys/unpcb.h
	Delete the obsolete unp_cc and unp_mbcnt fields from struct unpcb.

sys/kern/uipc_usrreq.c
	Adjust uipc_rcvd, uipc_send, and uipc_sense to use the SB_STOP
	backpressure mechanism.  Removing obsolete unpcb fields from
	db_show_unpcb.

tests/sys/kern/unix_seqpacket_test.c
	Clear expected failures from ATF.

Obtained from:	DragonFly BSD
PR:		kern/185812
Reviewed by:	silence from freebsd-net@ and rwatson@
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corporation

1 files changed, 14 insertions, 3 deletions

diff --git a/sys/sys/sockbuf.h b/sys/sys/sockbuf.h
index 6994fd3..877b530 100644
--- a/sys/sys/sockbuf.h
+++ b/sys/sys/sockbuf.h
@@ -52,6 +52,7 @@
 #define	SB_NOCOALESCE	0x200		/* don't coalesce new data into existing mbufs */
 #define	SB_IN_TOE	0x400		/* socket buffer is in the middle of an operation */
 #define	SB_AUTOSIZE	0x800		/* automatically size socket buffer */
+#define	SB_STOP		0x1000		/* backpressure indicator */
 
 #define	SBS_CANTSENDMORE	0x0010	/* can't send more data to peer */
 #define	SBS_CANTRCVMORE		0x0020	/* can't receive more data from peer */
@@ -168,9 +169,19 @@ void	sbunlock(struct sockbuf *sb);
  * still be negative (cc > hiwat or mbcnt > mbmax).  Should detect
  * overflow and return 0.  Should use "lmin" but it doesn't exist now.
  */
-#define	sbspace(sb) \
-    ((long) imin((int)((sb)->sb_hiwat - (sb)->sb_cc), \
-	 (int)((sb)->sb_mbmax - (sb)->sb_mbcnt)))
+static __inline
+long
+sbspace(struct sockbuf *sb)
+{
+	long bleft;
+	long mleft;
+
+	if (sb->sb_flags & SB_STOP)
+		return(0);
+	bleft = sb->sb_hiwat - sb->sb_cc;
+	mleft = sb->sb_mbmax - sb->sb_mbcnt;
+	return((bleft < mleft) ? bleft : mleft);
+}
 
 /* adjust counters in sb reflecting allocation of m */
 #define	sballoc(sb, m) { \