Replace the code for reading and writing the kernel message buffer

with a new implementation that has a mostly reentrant "addchar"
routine, supports multiple message buffers in the kernel, and hides
the implementation details from callers.

The new code uses a kind of sequence number to represend the current
read and write positions in the buffer. This approach (suggested
mainly by bde) permits the read and write pointers to be maintained
separately, which reduces the number of atomic operations that are
required. The "mostly reentrant" above refers to the way that while
it is now always safe to have any number of concurrent writers,
readers could see the message buffer after a writer has advanced
the pointers but before it has witten the new character.

Discussed on:	freebsd-arch

1 files changed, 22 insertions, 4 deletions

diff --git a/sys/sys/msgbuf.h b/sys/sys/msgbuf.h
index 22bd161..b92ccc8 100644
--- a/sys/sys/msgbuf.h
+++ b/sys/sys/msgbuf.h
@@ -38,19 +38,37 @@
 #define _SYS_MSGBUF_H_
 
 struct msgbuf {
+	char	*msg_ptr;		/* pointer to buffer */
 #define	MSG_MAGIC	0x063062
 	u_int	msg_magic;
-	int	msg_size;		/* size of buffer area */
-	int	msg_bufx;		/* write pointer */
-	int	msg_bufr;		/* read pointer */
-	char	*msg_ptr;		/* pointer to buffer */
+	u_int	msg_size;		/* size of buffer area */
+	u_int	msg_wseq;		/* write sequence number */
+	u_int	msg_rseq;		/* read sequence number */
 	u_int	msg_cksum;		/* checksum of contents */
+	u_int	msg_seqmod;		/* range for sequence numbers */
 };
 
+/* Normalise a sequence number or a difference between sequence numbers */
+#define MSGBUF_SEQNORM(mbp, seq) (((seq) + (mbp)->msg_seqmod) % \
+    (mbp)->msg_seqmod)
+#define MSGBUF_SEQ_TO_POS(mbp, seq) ((seq) % (mbp)->msg_size)
+/* Subtract sequence numbers, but note that only positive values result. */
+#define MSGBUF_SEQSUB(mbp, seq1, seq2) (MSGBUF_SEQNORM((mbp), (seq1) - (seq2)))
+
 #ifdef _KERNEL
 extern int	msgbuftrigger;
 extern struct	msgbuf *msgbufp;
 void	msgbufinit(void *ptr, int size);
+void	msgbuf_addchar(struct msgbuf *mbp, int c);
+void	msgbuf_clear(struct msgbuf *mbp);
+void	msgbuf_copy(struct msgbuf *src, struct msgbuf *dst);
+int	msgbuf_getbytes(struct msgbuf *mbp, char *buf, int buflen);
+int	msgbuf_getchar(struct msgbuf *mbp);
+int	msgbuf_getcount(struct msgbuf *mbp);
+void	msgbuf_init(struct msgbuf *mbp, void *ptr, int size);
+void	msgbuf_reinit(struct msgbuf *mbp, void *ptr, int size);
+int	msgbuf_peekbytes(struct msgbuf *mbp, char *buf, int buflen,
+	    u_int *seqp);
 
 #if !defined(MSGBUF_SIZE)
 #define	MSGBUF_SIZE	32768