Simon J. Gerraty wrote:
> I originally wrote kern_sem.c et al to provide interlocking for IPI's
> on sparc mp, which now have their own light weight interlocking.
> 
> Does anyone see any need for kernel semaphores in other areas?
> and if so, care to review the implementation below.  It caters for 
> multiple resource at a time allocations, and provides sleep and spin
> waits.   The hard bits are done using the existing locking primitives.

Would it be useful for implementation of e.g. POSIX (userland) semaphores,
or is it bound to be kernel-only?

Jaromir
 
> --sjg
> 
> 
> --- /dev/null	Mon Jan  7 01:32:24 2002
> +++ sys/sys/ksem.h	Tue Mar  6 10:11:15 2001
> @@ -0,0 +1,68 @@
> +/*	$NetBSD$	*/
> +
> +/*-
> + * Copyright (c) 2001 The NetBSD Foundation, Inc.
> + * All rights reserved.
> + *
> + * This code is derived from software contributed to The NetBSD Foundation
> + * by Simon Gerraty.
> + *
> + * 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. All advertising materials mentioning features or use of this software
> + *    must display the following acknowledgement:
> + *	This product includes software developed by the NetBSD
> + *	Foundation, Inc. and its contributors.
> + * 4. Neither the name of The NetBSD Foundation 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 NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
> + */
> +
> +#ifndef _SYS_KSEM_H_
> +# define _SYS_KSEM_H_
> +
> +#include <sys/lock.h>
> +
> +#ifdef _KERNEL
> +typedef struct sema_s {
> +	struct simplelock sem_interlock;
> +	const char *sem_wmesg;
> +	int	sem_flags;
> +	int	sem_sleepers;
> +	int	sem_count;
> +} sema_t;
> +
> +#define SEMAF_VALID (1<<0)		/* initialized */
> +#define SEMAF_DEBUG (1<<1)
> +
> +void	sema_init(sema_t *sp, int cnt, const char *wmesg, int flags);
> +void	sema_clear(sema_t *sp);		/* shutdown */
> +void	sema_setflags(sema_t *sp, int flags);
> +int	sema_signal(sema_t *sp);	/* V() */
> +int	sema_signal_n(sema_t *sp, int n); /* V() * n */
> +void	sema_spinwait(sema_t *sp, int n); /* spin version of P() * n */
> +void	sema_wait(sema_t *sp);		/* sleep version of P() */
> +void	sema_wait_n(sema_t *sp, int n);	/* sleep version of P() * n */
> +
> +#define sema_signal(sp) sema_signal_n((sp), 1)
> +
> +#endif
> +#endif
> --- /dev/null	Mon Jan  7 01:32:24 2002
> +++ sys/kern/kern_sem.c	Mon Jan  7 01:26:09 2002
> @@ -0,0 +1,303 @@
> +/*	$NetBSD$	*/
> +
> +/*-
> + * Copyright (c) 2001 The NetBSD Foundation, Inc.
> + * All rights reserved.
> + *
> + * This code is derived from software contributed to The NetBSD Foundation
> + * by Simon Gerraty.
> + *
> + * 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. All advertising materials mentioning features or use of this software
> + *    must display the following acknowledgement:
> + *	This product includes software developed by the NetBSD
> + *	Foundation, Inc. and its contributors.
> + * 4. Neither the name of The NetBSD Foundation 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 NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
> + */
> +
> +#include <sys/param.h>
> +#include <sys/systm.h>
> +#include <sys/proc.h>
> +#include <sys/ksem.h>
> +
> +#define SEMADEBUG
> +#if defined(LOCKDEBUG) && !defined(SEMADEBUG)
> +# define SEMADEBUG
> +#endif
> +
> +#ifdef SEMADEBUG
> +int sema_debug = 1;
> +#endif
> +
> +/*
> + * XXX not at all sure we should be using spl* here.
> + * so make it easy to undo.
> + */
> +//#define SEMASPL splhigh
> +#ifdef SEMASPL
> +# define SEMA_SPL(x) x
> +#else
> +# define SEMA_SPL(x)
> +#endif
> +
> +void
> +sema_init (sema_t *sp, int cnt, const char *wmesg, int flags)
> +{
> +	simple_lock_init(&sp->sem_interlock);
> +	sp->sem_wmesg = wmesg;
> +	sp->sem_count = cnt;
> +	sp->sem_sleepers = 0;
> +	sp->sem_flags = SEMAF_VALID|flags;
> +#ifdef SEMADEBUG
> +	if (sema_debug < 0 || (sema_debug && (flags & SEMAF_DEBUG))) {
> +		printf("\n{%d}sema_init(%p, %d, %s)\n", cpu_number(),
> +		       sp, cnt, wmesg ?: "");
> +		if (sema_debug < 0)
> +			sema_debug = 0;
> +	}
> +#endif
> +}
> +
> +void
> +sema_clear (sema_t *sp)
> +{
> +	SEMA_SPL(int s);
> +	
> +	if ((sp->sem_flags & SEMAF_VALID)) {
> +#ifdef SEMADEBUG
> +		if (sema_debug && (sp->sem_flags & SEMAF_DEBUG))
> +			printf("\n{%d}sema_clear(%p) count==%d, sleepers==%d\n",
> +			       cpu_number(),
> +			       sp, sp->sem_count, sp->sem_sleepers);
> +#endif
> +		SEMA_SPL(s = SEMASPL());
> +		simple_lock(&sp->sem_interlock);
> +		sp->sem_count = sp->sem_flags = 0;
> +		/*
> +		 * Its important that no-one be sleeping on the semaphore
> +		 * since after this, no one will ever wake them up.
> +		 */
> +		while (sp->sem_sleepers > 0) {
> +#ifdef DIAGNOSTIC
> +			panic("sema_clear: sleepers > 0");
> +#endif
> +			sp->sem_sleepers--;
> +			wakeup_one(sp);
> +		}
> +		simple_unlock(&sp->sem_interlock);
> +		SEMA_SPL(splx(s));
> +	}
> +}
> +
> +void
> +sema_setflags (sema_t *sp, int flags)
> +{
> +#ifdef SEMADEBUG
> +	if (sema_debug && (sp->sem_flags & SEMAF_DEBUG))
> +		printf("\n{%d}sema_setflags(%p, %d)\n", cpu_number(),
> +		       sp, flags);
> +#endif
> +	if ((flags & SEMAF_VALID) == 0) {
> +		sema_clear(sp);
> +	}
> +	sp->sem_flags = flags;
> +}
> +
> +
> +/*
> + * This is the normal wait routine.
> + * If decrementing the count takes it below 0 we
> + * sleep until sema_signal() ups to to 0 and awakens us.
> + * We don't make sema_wait() just sema_wait_n() with n=1 since
> + * this is a little more efficient.
> + */
> +void
> +sema_wait (sema_t *sp)
> +{
> +	struct proc *p = curproc;
> +	SEMA_SPL(int s);
> +	
> +	if (!(sp->sem_flags & SEMAF_VALID) || panicstr != NULL) {
> +		return;
> +	}
> +	SEMA_SPL(s = SEMASPL());
> +	simple_lock(&sp->sem_interlock);
> +	sp->sem_count--;
> +
> +#ifdef SEMADEBUG
> +	if (sema_debug && (sp->sem_flags & SEMAF_DEBUG))
> +		printf("\n{%d}sema_wait(%p) == %d\n",
> +		       cpu_number(),
> +		       sp, sp->sem_count);
> +#endif
> +	if (sp->sem_count < 0) {
> +		sp->sem_sleepers++;
> +		ltsleep(sp, p->p_priority|PNORELOCK,
> +			sp->sem_wmesg ?: "sema",
> +			0, &sp->sem_interlock);
> +		sp->sem_sleepers--;
> +		SEMA_SPL(splx(s));
> +		return;
> +	}
> +	simple_unlock(&sp->sem_interlock);
> +	SEMA_SPL(splx(s));
> +}
> +
> +
> +/*
> + * Like sema_wait() but allows reserving n resources at once.
> + * We need to loop around the sleep call, to ensure that we hold the
> + * interlock and that there are still "n" resources available.
> + * Unlike sema_wait() we have ltsleep() re-aquire the interlock when
> + * we wakeup.
> + */
> +void
> +sema_wait_n (sema_t *sp, int n)
> +{
> +	struct proc *p = curproc;
> +	SEMA_SPL(int s);
> +	
> +	if (!(sp->sem_flags & SEMAF_VALID) || panicstr != NULL) {
> +		return;
> +	}
> +	SEMA_SPL(s = SEMASPL());
> +	simple_lock(&sp->sem_interlock);
> +
> +#ifdef SEMADEBUG
> +	if (sema_debug && (sp->sem_flags & SEMAF_DEBUG))
> +		printf("\n{%d}sema_wait_n(%p,%d) == %d\n",
> +		       cpu_number(),
> +		       sp, n, sp->sem_count);
> +#endif
> +	/*
> +	 * If there are less than "n" available, sleep until that changes.
> +	 */
> +	if (sp->sem_count - n < 0) {
> +		do {
> +			sp->sem_sleepers++;
> +			ltsleep(sp, p->p_priority,
> +				sp->sem_wmesg ?: "seman",
> +				0, &sp->sem_interlock);
> +			sp->sem_sleepers--;
> +			/*
> +			 * There is no guarantee that there are "n" available
> +			 * yet, so we may have to sleep again.
> +			 */
> +		} while (panicstr == NULL &&
> +			 (sp->sem_flags & SEMAF_VALID) &&
> +			 sp->sem_count - n < 0);
> +	}
> +	sp->sem_count -= n;		/* got them */
> +	simple_unlock(&sp->sem_interlock);
> +	SEMA_SPL(splx(s));
> +}
> +
> +/*
> + * Spinning version of sema_wait_n().
> + */
> +void
> +sema_spinwait (sema_t *sp, int n)
> +{
> +	SEMA_SPL(int s);
> +
> +	if (!(sp->sem_flags & SEMAF_VALID) || panicstr != NULL) {
> +		return;
> +	}
> +	SEMA_SPL(s = SEMASPL());
> +	simple_lock(&sp->sem_interlock);
> +
> +#ifdef SEMADEBUG
> +	if (sema_debug && (sp->sem_flags & SEMAF_DEBUG))
> +		printf("\n{%d}sema_spinwait(%p,%d) == %d\n",
> +		       cpu_number(),
> +		       sp, n, sp->sem_count);
> +#endif
> +	if (sp->sem_count - n < 0) {
> +		do {
> +			simple_unlock(&sp->sem_interlock);
> +			SEMA_SPL(splx(s));
> +
> +			while (panicstr == NULL &&
> +			       (sp->sem_flags & SEMAF_VALID) &&
> +			       sp->sem_count - n < 0)
> +				continue; /* spin */
> +			if (!(sp->sem_flags & SEMAF_VALID) ||
> +			    panicstr != NULL) {
> +				return;
> +			}
> +			
> +			SEMA_SPL(s = SEMASPL());
> +			simple_lock(&sp->sem_interlock);
> +		} while (panicstr == NULL && sp->sem_count - n < 0);
> +	}
> +	sp->sem_count -= n;		/* got them */
> +	simple_unlock(&sp->sem_interlock);
> +	SEMA_SPL(splx(s));
> +}
> +
> +
> +/*
> + * sema_signal() is just this routine with n=1
> + * If we find count>=0 we wake up at most "n" sleepers.
> + */
> +int
> +sema_signal_n (sema_t *sp, int n)
> +{
> +	int rc = 0;
> +	int i;
> +	SEMA_SPL(int s);
> +	
> +	if (panicstr == NULL && (sp->sem_flags & SEMAF_VALID)) {
> +		SEMA_SPL(s = SEMASPL());
> +		simple_lock(&sp->sem_interlock);
> +		sp->sem_count += n;
> +
> +#ifdef SEMADEBUG
> +		if (sema_debug && (sp->sem_flags & SEMAF_DEBUG))
> +			printf("\n{%d}sema_signal(%p,%d) == %d\n",
> +			       cpu_number(),
> +			       sp, n, sp->sem_count);
> +#endif
> +		if ((rc = sp->sem_count) >= 0 && sp->sem_sleepers > 0) {
> +			/*
> +			 * Wakeup at most n sleepers.
> +			 */
> +			for (i = 0; i < n && sp->sem_count - i >= 0; ++i) {
> +#ifdef SEMADEBUG
> +				if (sema_debug &&
> +				    (sp->sem_flags & SEMAF_DEBUG))
> +					printf("\n{%d}sema_signal: wakeup %d\n",
> +					       cpu_number(), i);
> +#endif
> +
> +				wakeup_one(sp);
> +			}
> +		}
> +		simple_unlock(&sp->sem_interlock);
> +		SEMA_SPL(splx(s));
> +	}
> +	return rc;
> +}
> +
> +
> Index: sys/conf/files
> ===================================================================
> RCS file: /cvsroot/syssrc/sys/conf/files,v
> retrieving revision 1.486
> diff -u -p -r1.486 files
> --- sys/conf/files	2001/12/17 15:40:43	1.486
> +++ sys/conf/files	2002/01/07 09:36:22
> @@ -1056,6 +1056,7 @@ file	kern/kern_physio.c
>  file	kern/kern_proc.c
>  file	kern/kern_prot.c
>  file	kern/kern_resource.c
> +file	kern/kern_sem.c
>  file	kern/kern_sig.c
>  file	kern/kern_subr.c
>  file	kern/kern_synch.c
> 


-- 
Jaromir Dolecek <jdolecek@NetBSD.org> http://www.NetBSD.org/Ports/i386/ps2.html
-=  Those who would give up liberty for a little temporary safety deserve  =-
-=  neither liberty nor safety, and will lose both.  -- Benjamin Franklin  =-