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Serialize msgbuf access with a mutex.

This introduces a system-wide mutex that serializes msgbuf operations.
The mutex controls access to all modifiable fields of struct msgbuf.
It also covers logsoftc.sc_state.

To avoid adding extra lock order constraints that would affect use of
printf(9), the code does not take new locks when the log mutex is held.

The code assumes that there is at most one thread using logread(). This
keeps the logic simple. If there was more than one reader, logread()
might return the same data to different readers. Also, log wakeup might
not be reliable with multiple threads.

Tested in snaps for two weeks.

OK mpi@

/*	$OpenBSD: msgbuf.h,v 1.13 2020/10/25 10:55:42 visa Exp $	*/
/*	$NetBSD: msgbuf.h,v 1.8 1995/03/26 20:24:27 jtc Exp $	*/

/*
 * Copyright (c) 1981, 1984, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)msgbuf.h	8.1 (Berkeley) 6/2/93
 */

/*
 * Locking:
 *	I	immutable after creation
 *	L	log_mtx
 *	Lw	log_mtx for writing
 */
struct	msgbuf {
#define	MSG_MAGIC	0x063061
	long	msg_magic;		/* [I] buffer magic value */
	long	msg_bufx;		/* [L] write pointer */
	long	msg_bufr;		/* [L] read pointer */
	long	msg_bufs;		/* [I] real msg_bufc size (bytes) */
	long	msg_bufd;		/* [L] number of dropped bytes */
	char	msg_bufc[1];		/* [Lw] buffer */
};
#ifdef _KERNEL
#define CONSBUFSIZE	(16 * 1024)	/* console message buffer size */
extern struct msgbuf *msgbufp;
extern struct msgbuf *consbufp;

void	initmsgbuf(caddr_t buf, size_t bufsize);
void	initconsbuf(void);
void	msgbuf_putchar(struct msgbuf *, const char c);
#endif