Fix cblock starvation bugs by reserving enough cblocks for minimal

operation of each clist.  Limit the growth of each clist.  Clists
can only grow larger than the reserved minimum if there are free
cblocks in a shared pool.  The size of this pool is now fixed
(this could be improved).  The reserved and maximum sizes are more
carefully allocated for slip and ppp, depending on the mtu.  A maximum
MTU of 16384 is now enforced for ppp.

3 files changed, 279 insertions, 146 deletions

diff --git a/sys/kern/subr_clist.c b/sys/kern/subr_clist.c
index 46a8f1e..a261533 100644
--- a/sys/kern/subr_clist.c
+++ b/sys/kern/subr_clist.c
@@ -6,7 +6,7 @@
  *   of this software, nor does the author assume any responsibility
  *   for damages incurred with its use.
  *
- * $Id: tty_subr.c,v 1.7 1994/09/25 19:33:50 phk Exp $
+ * $Id: tty_subr.c,v 1.8 1994/10/30 19:43:49 bde Exp $
  */
 
 /*
@@ -22,17 +22,25 @@
 
 struct cblock *cfreelist = 0;
 int cfreecount = 0;
+static int cslushcount;
+static int ctotcount;
 
 #ifndef INITIAL_CBLOCKS
-#define INITIAL_CBLOCKS 50
+#define	INITIAL_CBLOCKS 50
 #endif
 
-#define MBUF_DIAG
-#ifdef MBUF_DIAG
-void
-print_nblocks()
+static void cblock_alloc_cblocks __P((int number));
+static void cblock_free_cblocks __P((int number));
+
+#define	CBLOCK_DIAG
+#ifdef CBLOCK_DIAG
+static void
+cbstat()
 {
-	printf("There are currently %d bytes in cblocks\n", cfreecount);
+	printf(
+	"tot = %d (active = %d, free = %d (reserved = %d, slush = %d))\n",
+	       ctotcount * CBSIZE, ctotcount * CBSIZE - cfreecount, cfreecount,
+	       cfreecount - cslushcount * CBSIZE, cslushcount * CBSIZE);
 }
 #endif
 
@@ -44,10 +52,13 @@ clist_init()
 {
 	/*
 	 * Allocate an initial base set of cblocks as a 'slush'.
-	 * We allocate more with each ttyopen().
+	 * We allocate non-slush cblocks with each initial ttyopen() and
+	 * deallocate them with each ttyclose().
+	 * We should adjust the slush allocation.  This can't be done in
+	 * the i/o routines because they are sometimes called from
+	 * interrupt handlers when it may be unsafe to call malloc().
 	 */
-	cblock_alloc_cblocks(INITIAL_CBLOCKS);
-	return;
+	cblock_alloc_cblocks(cslushcount = INITIAL_CBLOCKS);
 }
 
 /*
@@ -60,10 +71,8 @@ cblock_alloc()
 	struct cblock *cblockp;
 
 	cblockp = cfreelist;
-	if (!cblockp) {
-		/* XXX should syslog a message that we're out! */
-		return (0);
-	}
+	if (cblockp == NULL)
+		panic("clist reservation botch");
 	cfreelist = cblockp->c_next;
 	cblockp->c_next = NULL;
 	cfreecount -= CBSIZE;
@@ -80,13 +89,12 @@ cblock_free(cblockp)
 	cblockp->c_next = cfreelist;
 	cfreelist = cblockp;
 	cfreecount += CBSIZE;
-	return;
 }
 
 /*
  * Allocate some cblocks for the cfreelist queue.
  */
-void
+static void
 cblock_alloc_cblocks(number)
 	int number;
 {
@@ -94,36 +102,66 @@ cblock_alloc_cblocks(number)
 	struct cblock *tmp;
 
 	for (i = 0; i < number; ++i) {
-		tmp = malloc(sizeof(struct cblock), M_TTYS, M_NOWAIT);
-		if (!tmp) {
-			printf("cblock_alloc_cblocks: could not malloc cblock");
-			break;
-		}
+		tmp = malloc(sizeof(struct cblock), M_TTYS, M_WAITOK);
 		bzero((char *)tmp, sizeof(struct cblock));
 		cblock_free(tmp);
 	}
-	return;
+	ctotcount += number;
+}
+
+/*
+ * Set the cblock allocation policy for a a clist.
+ * Must be called at spltty().
+ */
+void
+clist_alloc_cblocks(clistp, ccmax, ccreserved)
+	struct clist *clistp;
+	int ccmax;
+	int ccreserved;
+{
+	int dcbr;
+
+	clistp->c_cbmax = roundup(ccmax, CBSIZE) / CBSIZE;
+	dcbr = roundup(ccreserved, CBSIZE) / CBSIZE - clistp->c_cbreserved;
+	if (dcbr >= 0)
+		cblock_alloc_cblocks(dcbr);
+	else {
+		if (clistp->c_cbreserved + dcbr < clistp->c_cbcount)
+			dcbr = clistp->c_cbcount - clistp->c_cbreserved;
+		cblock_free_cblocks(-dcbr);
+	}
+	clistp->c_cbreserved += dcbr;
 }
 
 /*
  * Free some cblocks from the cfreelist queue back to the
  * system malloc pool.
  */
-void
+static void
 cblock_free_cblocks(number)
 	int number;
 {
 	int i;
-	struct cblock *tmp;
 
-	for (i = 0; i < number; ++i) {
-		tmp = cblock_alloc();
-		if (tmp == NULL)
-			return;
-		free(tmp, M_TTYS);
-	}
+	for (i = 0; i < number; ++i)
+		free(cblock_alloc(), M_TTYS);
+	ctotcount -= number;
 }
 
+/*
+ * Free the cblocks reserved for a clist.
+ * Must be called at spltty().
+ */
+void
+clist_free_cblocks(clistp)
+	struct clist *clistp;
+{
+	if (clistp->c_cbcount != 0)
+		panic("freeing active clist cblocks");
+	cblock_free_cblocks(clistp->c_cbreserved);
+	clistp->c_cbmax = 0;
+	clistp->c_cbreserved = 0;
+}
 
 /*
  * Get a character from the head of a clist.
@@ -168,6 +206,8 @@ getc(clistp)
 				clistp->c_cf = clistp->c_cl = NULL;
 			}
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 		}
 	}
 
@@ -217,6 +257,8 @@ q_to_b(clistp, dest, amount)
 				clistp->c_cf = clistp->c_cl = NULL;
 			}
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 		}
 	}
 
@@ -260,11 +302,12 @@ ndflush(clistp, amount)
 				clistp->c_cf = clistp->c_cl = NULL;
 			}
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 		}
 	}
 
 	splx(s);
-	return;
 }
 
 /*
@@ -281,28 +324,30 @@ putc(chr, clistp)
 
 	s = spltty();
 
-	cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
-
 	if (clistp->c_cl == NULL) {
+		if (clistp->c_cbreserved < 1)
+			panic("putc to a clist with no reserved cblocks");
 		cblockp = cblock_alloc();
-		if (cblockp) {
-			clistp->c_cf = clistp->c_cl = cblockp->c_info;
-			clistp->c_cc = 0;
-		} else {
-			splx(s);
-			return (-1);
-		}
+		clistp->c_cbcount = 1;
+		clistp->c_cf = clistp->c_cl = cblockp->c_info;
+		clistp->c_cc = 0;
 	} else {
+		cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
 		if (((long)clistp->c_cl & CROUND) == 0) {
 			struct cblock *prev = (cblockp - 1);
-			cblockp = cblock_alloc();
-			if (cblockp) {
-				prev->c_next = cblockp;
-				clistp->c_cl = cblockp->c_info;
-			} else {
-				splx(s);
-				return (-1);
+
+			if (clistp->c_cbcount >= clistp->c_cbreserved) {
+				if (clistp->c_cbcount >= clistp->c_cbmax
+				    || cslushcount <= 0) {
+					splx(s);
+					return (-1);
+				}
+				--cslushcount;
 			}
+			cblockp = cblock_alloc();
+			clistp->c_cbcount++;
+			prev->c_next = cblockp;
+			clistp->c_cl = cblockp->c_info;
 		}
 	}
 
@@ -337,6 +382,13 @@ b_to_q(src, amount, clistp)
 	int startbit, endbit, num_between, numc;
 	int s;
 
+	/*
+	 * Avoid allocating an initial cblock and then not using it.
+	 * c_cc == 0 must imply c_cbount == 0.
+	 */
+	if (amount <= 0)
+		return (amount);
+
 	s = spltty();
 
 	/*
@@ -344,14 +396,12 @@ b_to_q(src, amount, clistp)
 	 * then get one.
 	 */
 	if (clistp->c_cl == NULL) {
+		if (clistp->c_cbreserved < 1)
+			panic("b_to_q to a clist with no reserved cblocks");
 		cblockp = cblock_alloc();
-		if (cblockp) {
-			clistp->c_cf = clistp->c_cl = cblockp->c_info;
-			clistp->c_cc = 0;
-		} else {
-			splx(s);
-			return (amount);
-		}
+		clistp->c_cbcount = 1;
+		clistp->c_cf = clistp->c_cl = cblockp->c_info;
+		clistp->c_cc = 0;
 	} else {
 		cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
 	}
@@ -362,14 +412,19 @@ b_to_q(src, amount, clistp)
 		 */
 		if (((long)clistp->c_cl & CROUND) == 0) {
 			struct cblock *prev = cblockp - 1;
-			cblockp = cblock_alloc();
-			if (cblockp) {
-				prev->c_next = cblockp;
-				clistp->c_cl = cblockp->c_info;
-			} else {
-				splx(s);
-				return (amount);
+
+			if (clistp->c_cbcount >= clistp->c_cbreserved) {
+				if (clistp->c_cbcount >= clistp->c_cbmax
+				    || cslushcount <= 0) {
+					splx(s);
+					return (amount);
+				}
+				--cslushcount;
 			}
+			cblockp = cblock_alloc();
+			clistp->c_cbcount++;
+			prev->c_next = cblockp;
+			clistp->c_cl = cblockp->c_info;
 		}
 
 		/*
@@ -515,6 +570,8 @@ unputc(clistp)
 			 */
 			clistp->c_cl = (char *)(cbp+1);
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 			cbp->c_next = NULL;
 		}
 	}
@@ -526,6 +583,8 @@ unputc(clistp)
 	if ((clistp->c_cc == 0) && clistp->c_cl) {
 		cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
 		cblock_free(cblockp);
+		if (--clistp->c_cbcount >= clistp->c_cbreserved)
+			++cslushcount;
 		clistp->c_cf = clistp->c_cl = NULL;
 	}
 
@@ -546,19 +605,25 @@ catq(src_clistp, dest_clistp)
 	s = spltty();
 	/*
 	 * If the destination clist is empty (has no cblocks atttached),
+	 * and there are no possible complications with the resource counters,
 	 * then we simply assign the current clist to the destination.
 	 */
-	if (!dest_clistp->c_cf) {
+	if (!dest_clistp->c_cf
+	    && src_clistp->c_cbcount <= src_clistp->c_cbmax
+	    && src_clistp->c_cbcount <= dest_clistp->c_cbmax) {
 		dest_clistp->c_cf = src_clistp->c_cf;
 		dest_clistp->c_cl = src_clistp->c_cl;
 		src_clistp->c_cf = src_clistp->c_cl = NULL;
 
 		dest_clistp->c_cc = src_clistp->c_cc;
 		src_clistp->c_cc = 0;
+		dest_clistp->c_cbcount = src_clistp->c_cbcount;
+		src_clistp->c_cbcount = 0;
 
 		splx(s);
 		return;
 	}
+
 	splx(s);
 
 	/*
@@ -567,6 +632,4 @@ catq(src_clistp, dest_clistp)
 	 */
 	while ((chr = getc(src_clistp)) != -1)
 		putc(chr, dest_clistp);
-
-	return;
 }
diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index dca12f9..d15110f 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -36,7 +36,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tty.c	8.8 (Berkeley) 1/21/94
- * $Id: tty.c,v 1.13 1994/11/01 22:23:29 bde Exp $
+ * $Id: tty.c,v 1.14 1994/11/26 18:54:25 bde Exp $
  */
 
 #include <sys/param.h>
@@ -72,10 +72,6 @@ char ttybuf[]	= "ttybuf";
 char ttyin[]	= "ttyin";
 char ttyout[]	= "ttyout";
 
-#ifndef CBLOCKS_PER_TTY
-#define CBLOCKS_PER_TTY 10
-#endif
-
 /*
  * Table with character classes and parity. The 8th bit indicates parity,
  * the 7th bit indicates the character is an alphameric or underscore (for
@@ -169,12 +165,17 @@ ttyopen(device, tp)
 	if (!ISSET(tp->t_state, TS_ISOPEN)) {
 		SET(tp->t_state, TS_ISOPEN);
 		bzero(&tp->t_winsize, sizeof(tp->t_winsize));
-		/*
-		 * Add some cblocks to the clistfree pool.
-		 */
-		cblock_alloc_cblocks(CBLOCKS_PER_TTY);
 	}
 	CLR(tp->t_state, TS_WOPEN);
+
+	/*
+	 * Initialize or restore a cblock allocation policy suitable for
+	 * the standard line discipline.
+	 */
+	clist_alloc_cblocks(&tp->t_canq, TTYHOG, 512);
+	clist_alloc_cblocks(&tp->t_outq, TTMAXHIWAT + 200, 512);
+	clist_alloc_cblocks(&tp->t_rawq, TTYHOG, 512);
+
 	splx(s);
 	return (0);
 }
@@ -189,23 +190,22 @@ ttyclose(tp)
 	register struct tty *tp;
 {
 	extern struct tty *constty;	/* Temporary virtual console. */
+	int s;
 
+	s = spltty();
 	if (constty == tp)
 		constty = NULL;
 
 	ttyflush(tp, FREAD | FWRITE);
+	clist_free_cblocks(&tp->t_canq);
+	clist_free_cblocks(&tp->t_outq);
+	clist_free_cblocks(&tp->t_rawq);
 
 	tp->t_gen++;
 	tp->t_pgrp = NULL;
 	tp->t_session = NULL;
-	/*
-	 * If the tty has not already been closed, free the cblocks
-	 * that were allocated in ttyopen() back to the system malloc
-	 * pool.
-	 */
-	if (ISSET(tp->t_state, (TS_ISOPEN|TS_WOPEN)))
-		cblock_free_cblocks(CBLOCKS_PER_TTY);
 	tp->t_state = 0;
+	splx(s);
 	return (0);
 }
 
@@ -1209,9 +1209,16 @@ ttypend(tp)
 
 	CLR(tp->t_lflag, PENDIN);
 	SET(tp->t_state, TS_TYPEN);
+	/*
+	 * XXX this assumes too much about clist internals.  It may even
+	 * fail if the cblock slush pool is empty.  We can't allocate more
+	 * cblocks here because we are called from an interrupt handler
+	 * and clist_alloc_cblocks() can wait.
+	 */
 	tq = tp->t_rawq;
-	tp->t_rawq.c_cc = 0;
-	tp->t_rawq.c_cf = tp->t_rawq.c_cl = 0;
+	bzero(&tp->t_rawq, sizeof tp->t_rawq);
+	tp->t_rawq.c_cbmax = tq.c_cbmax;
+	tp->t_rawq.c_cbreserved = tq.c_cbreserved;
 	while ((c = getc(&tq)) >= 0)
 		ttyinput(c, tp);
 	CLR(tp->t_state, TS_TYPEN);
diff --git a/sys/kern/tty_subr.c b/sys/kern/tty_subr.c
index 46a8f1e..a261533 100644
--- a/sys/kern/tty_subr.c
+++ b/sys/kern/tty_subr.c
@@ -6,7 +6,7 @@
  *   of this software, nor does the author assume any responsibility
  *   for damages incurred with its use.
  *
- * $Id: tty_subr.c,v 1.7 1994/09/25 19:33:50 phk Exp $
+ * $Id: tty_subr.c,v 1.8 1994/10/30 19:43:49 bde Exp $
  */
 
 /*
@@ -22,17 +22,25 @@
 
 struct cblock *cfreelist = 0;
 int cfreecount = 0;
+static int cslushcount;
+static int ctotcount;
 
 #ifndef INITIAL_CBLOCKS
-#define INITIAL_CBLOCKS 50
+#define	INITIAL_CBLOCKS 50
 #endif
 
-#define MBUF_DIAG
-#ifdef MBUF_DIAG
-void
-print_nblocks()
+static void cblock_alloc_cblocks __P((int number));
+static void cblock_free_cblocks __P((int number));
+
+#define	CBLOCK_DIAG
+#ifdef CBLOCK_DIAG
+static void
+cbstat()
 {
-	printf("There are currently %d bytes in cblocks\n", cfreecount);
+	printf(
+	"tot = %d (active = %d, free = %d (reserved = %d, slush = %d))\n",
+	       ctotcount * CBSIZE, ctotcount * CBSIZE - cfreecount, cfreecount,
+	       cfreecount - cslushcount * CBSIZE, cslushcount * CBSIZE);
 }
 #endif
 
@@ -44,10 +52,13 @@ clist_init()
 {
 	/*
 	 * Allocate an initial base set of cblocks as a 'slush'.
-	 * We allocate more with each ttyopen().
+	 * We allocate non-slush cblocks with each initial ttyopen() and
+	 * deallocate them with each ttyclose().
+	 * We should adjust the slush allocation.  This can't be done in
+	 * the i/o routines because they are sometimes called from
+	 * interrupt handlers when it may be unsafe to call malloc().
 	 */
-	cblock_alloc_cblocks(INITIAL_CBLOCKS);
-	return;
+	cblock_alloc_cblocks(cslushcount = INITIAL_CBLOCKS);
 }
 
 /*
@@ -60,10 +71,8 @@ cblock_alloc()
 	struct cblock *cblockp;
 
 	cblockp = cfreelist;
-	if (!cblockp) {
-		/* XXX should syslog a message that we're out! */
-		return (0);
-	}
+	if (cblockp == NULL)
+		panic("clist reservation botch");
 	cfreelist = cblockp->c_next;
 	cblockp->c_next = NULL;
 	cfreecount -= CBSIZE;
@@ -80,13 +89,12 @@ cblock_free(cblockp)
 	cblockp->c_next = cfreelist;
 	cfreelist = cblockp;
 	cfreecount += CBSIZE;
-	return;
 }
 
 /*
  * Allocate some cblocks for the cfreelist queue.
  */
-void
+static void
 cblock_alloc_cblocks(number)
 	int number;
 {
@@ -94,36 +102,66 @@ cblock_alloc_cblocks(number)
 	struct cblock *tmp;
 
 	for (i = 0; i < number; ++i) {
-		tmp = malloc(sizeof(struct cblock), M_TTYS, M_NOWAIT);
-		if (!tmp) {
-			printf("cblock_alloc_cblocks: could not malloc cblock");
-			break;
-		}
+		tmp = malloc(sizeof(struct cblock), M_TTYS, M_WAITOK);
 		bzero((char *)tmp, sizeof(struct cblock));
 		cblock_free(tmp);
 	}
-	return;
+	ctotcount += number;
+}
+
+/*
+ * Set the cblock allocation policy for a a clist.
+ * Must be called at spltty().
+ */
+void
+clist_alloc_cblocks(clistp, ccmax, ccreserved)
+	struct clist *clistp;
+	int ccmax;
+	int ccreserved;
+{
+	int dcbr;
+
+	clistp->c_cbmax = roundup(ccmax, CBSIZE) / CBSIZE;
+	dcbr = roundup(ccreserved, CBSIZE) / CBSIZE - clistp->c_cbreserved;
+	if (dcbr >= 0)
+		cblock_alloc_cblocks(dcbr);
+	else {
+		if (clistp->c_cbreserved + dcbr < clistp->c_cbcount)
+			dcbr = clistp->c_cbcount - clistp->c_cbreserved;
+		cblock_free_cblocks(-dcbr);
+	}
+	clistp->c_cbreserved += dcbr;
 }
 
 /*
  * Free some cblocks from the cfreelist queue back to the
  * system malloc pool.
  */
-void
+static void
 cblock_free_cblocks(number)
 	int number;
 {
 	int i;
-	struct cblock *tmp;
 
-	for (i = 0; i < number; ++i) {
-		tmp = cblock_alloc();
-		if (tmp == NULL)
-			return;
-		free(tmp, M_TTYS);
-	}
+	for (i = 0; i < number; ++i)
+		free(cblock_alloc(), M_TTYS);
+	ctotcount -= number;
 }
 
+/*
+ * Free the cblocks reserved for a clist.
+ * Must be called at spltty().
+ */
+void
+clist_free_cblocks(clistp)
+	struct clist *clistp;
+{
+	if (clistp->c_cbcount != 0)
+		panic("freeing active clist cblocks");
+	cblock_free_cblocks(clistp->c_cbreserved);
+	clistp->c_cbmax = 0;
+	clistp->c_cbreserved = 0;
+}
 
 /*
  * Get a character from the head of a clist.
@@ -168,6 +206,8 @@ getc(clistp)
 				clistp->c_cf = clistp->c_cl = NULL;
 			}
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 		}
 	}
 
@@ -217,6 +257,8 @@ q_to_b(clistp, dest, amount)
 				clistp->c_cf = clistp->c_cl = NULL;
 			}
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 		}
 	}
 
@@ -260,11 +302,12 @@ ndflush(clistp, amount)
 				clistp->c_cf = clistp->c_cl = NULL;
 			}
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 		}
 	}
 
 	splx(s);
-	return;
 }
 
 /*
@@ -281,28 +324,30 @@ putc(chr, clistp)
 
 	s = spltty();
 
-	cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
-
 	if (clistp->c_cl == NULL) {
+		if (clistp->c_cbreserved < 1)
+			panic("putc to a clist with no reserved cblocks");
 		cblockp = cblock_alloc();
-		if (cblockp) {
-			clistp->c_cf = clistp->c_cl = cblockp->c_info;
-			clistp->c_cc = 0;
-		} else {
-			splx(s);
-			return (-1);
-		}
+		clistp->c_cbcount = 1;
+		clistp->c_cf = clistp->c_cl = cblockp->c_info;
+		clistp->c_cc = 0;
 	} else {
+		cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
 		if (((long)clistp->c_cl & CROUND) == 0) {
 			struct cblock *prev = (cblockp - 1);
-			cblockp = cblock_alloc();
-			if (cblockp) {
-				prev->c_next = cblockp;
-				clistp->c_cl = cblockp->c_info;
-			} else {
-				splx(s);
-				return (-1);
+
+			if (clistp->c_cbcount >= clistp->c_cbreserved) {
+				if (clistp->c_cbcount >= clistp->c_cbmax
+				    || cslushcount <= 0) {
+					splx(s);
+					return (-1);
+				}
+				--cslushcount;
 			}
+			cblockp = cblock_alloc();
+			clistp->c_cbcount++;
+			prev->c_next = cblockp;
+			clistp->c_cl = cblockp->c_info;
 		}
 	}
 
@@ -337,6 +382,13 @@ b_to_q(src, amount, clistp)
 	int startbit, endbit, num_between, numc;
 	int s;
 
+	/*
+	 * Avoid allocating an initial cblock and then not using it.
+	 * c_cc == 0 must imply c_cbount == 0.
+	 */
+	if (amount <= 0)
+		return (amount);
+
 	s = spltty();
 
 	/*
@@ -344,14 +396,12 @@ b_to_q(src, amount, clistp)
 	 * then get one.
 	 */
 	if (clistp->c_cl == NULL) {
+		if (clistp->c_cbreserved < 1)
+			panic("b_to_q to a clist with no reserved cblocks");
 		cblockp = cblock_alloc();
-		if (cblockp) {
-			clistp->c_cf = clistp->c_cl = cblockp->c_info;
-			clistp->c_cc = 0;
-		} else {
-			splx(s);
-			return (amount);
-		}
+		clistp->c_cbcount = 1;
+		clistp->c_cf = clistp->c_cl = cblockp->c_info;
+		clistp->c_cc = 0;
 	} else {
 		cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
 	}
@@ -362,14 +412,19 @@ b_to_q(src, amount, clistp)
 		 */
 		if (((long)clistp->c_cl & CROUND) == 0) {
 			struct cblock *prev = cblockp - 1;
-			cblockp = cblock_alloc();
-			if (cblockp) {
-				prev->c_next = cblockp;
-				clistp->c_cl = cblockp->c_info;
-			} else {
-				splx(s);
-				return (amount);
+
+			if (clistp->c_cbcount >= clistp->c_cbreserved) {
+				if (clistp->c_cbcount >= clistp->c_cbmax
+				    || cslushcount <= 0) {
+					splx(s);
+					return (amount);
+				}
+				--cslushcount;
 			}
+			cblockp = cblock_alloc();
+			clistp->c_cbcount++;
+			prev->c_next = cblockp;
+			clistp->c_cl = cblockp->c_info;
 		}
 
 		/*
@@ -515,6 +570,8 @@ unputc(clistp)
 			 */
 			clistp->c_cl = (char *)(cbp+1);
 			cblock_free(cblockp);
+			if (--clistp->c_cbcount >= clistp->c_cbreserved)
+				++cslushcount;
 			cbp->c_next = NULL;
 		}
 	}
@@ -526,6 +583,8 @@ unputc(clistp)
 	if ((clistp->c_cc == 0) && clistp->c_cl) {
 		cblockp = (struct cblock *)((long)clistp->c_cl & ~CROUND);
 		cblock_free(cblockp);
+		if (--clistp->c_cbcount >= clistp->c_cbreserved)
+			++cslushcount;
 		clistp->c_cf = clistp->c_cl = NULL;
 	}
 
@@ -546,19 +605,25 @@ catq(src_clistp, dest_clistp)
 	s = spltty();
 	/*
 	 * If the destination clist is empty (has no cblocks atttached),
+	 * and there are no possible complications with the resource counters,
 	 * then we simply assign the current clist to the destination.
 	 */
-	if (!dest_clistp->c_cf) {
+	if (!dest_clistp->c_cf
+	    && src_clistp->c_cbcount <= src_clistp->c_cbmax
+	    && src_clistp->c_cbcount <= dest_clistp->c_cbmax) {
 		dest_clistp->c_cf = src_clistp->c_cf;
 		dest_clistp->c_cl = src_clistp->c_cl;
 		src_clistp->c_cf = src_clistp->c_cl = NULL;
 
 		dest_clistp->c_cc = src_clistp->c_cc;
 		src_clistp->c_cc = 0;
+		dest_clistp->c_cbcount = src_clistp->c_cbcount;
+		src_clistp->c_cbcount = 0;
 
 		splx(s);
 		return;
 	}
+
 	splx(s);
 
 	/*
@@ -567,6 +632,4 @@ catq(src_clistp, dest_clistp)
 	 */
 	while ((chr = getc(src_clistp)) != -1)
 		putc(chr, dest_clistp);
-
-	return;
 }