xref: /src/sys/arch/arm/include/mutex.h

/*	$NetBSD: mutex.h,v 1.1 2007/02/18 07:24:52 matt Exp $	*/

/*-
 * Copyright (c) 2002, 2007 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe and Andrew Doran.
 *
 * 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 _ARM_MUTEX_H_
#define	_ARM_MUTEX_H_

/*
 * The arm mutex implementation is troublesome, because arm lacks a
 * compare-and-set operation.  However, there aren't MP arms.
 * SMP for spin mutexes is easy - we don't need
 * to know who owns the lock.  For adaptive mutexes, we need an aditional
 * interlock.
 *
 * The locked byte set by the sparc 'ldstub' instruction is 0xff.  sparc
 * kernels are always loaded above 0xe0000000, and the low 5 bits of any
 * "struct lwp *" are always zero.  So, to record the lock owner, we only
 * need 23 bits of space.  mtxa_owner contains the mutex owner's address
 * shifted right by 5: the top three bits of which will always be 0xe,
 * overlapping with the interlock at the top byte, which is always 0xff
 * when the mutex is held.
 *
 * For a mutex acquisition, the owner field is set in two steps: first,
 * acquire the interlock (top byte), and then test the owner's address.
 * Once the owner field is non zero, it will appear that the mutex is held,
 * by which LWP it does not matter: other LWPs competing for the lock will
 * fall through to mutex_vector_enter(), and either spin or sleep.
 *
 * As a result there is no space for a waiters bit in the owner field.  No
 * problem, because it would be hard to synchronise using one without a CAS
 * operation.  Note that in order to do unlocked release of adaptive
 * mutexes, we need the effect of MUTEX_SET_WAITERS() to be immediatley
 * visible on the bus.  So, adaptive mutexes share the spin lock byte with
 * spin mutexes (set with ldstub), but it is not treated as a lock in its
 * own right, rather as a flag that can be atomically set or cleared.
 *
 * When releasing an adaptive mutex, we first clear the owners field, and
 * then check to see if the waiters byte is set.  This ensures that there
 * will always be someone to wake any sleeping waiters up (even it the mutex
 * is acquired immediately after we release it, or if we are preempted
 * immediatley after clearing the owners field).  The setting or clearing of
 * the waiters byte is serialized by the turnstile chain lock associated
 * with the mutex.
 *
 * See comments in kern_mutex.c about releasing adaptive mutexes without
 * an interlocking step.
 */

#ifndef __MUTEX_PRIVATE

struct kmutex {
	uintptr_t	mtx_pad1;
	uint32_t	mtx_pad2[4];
};

#else	/* __MUTEX_PRIVATE */

struct kmutex {
	volatile uintptr_t	mtx_owner;		/* 0-3 */
	__cpu_simple_lock_t	mtx_interlock;		/* 4-7 */
	ipl_cookie_t		mtx_ipl;		/* 8-11 */
	__cpu_simple_lock_t	mtx_lock;		/* 12-15 */
	uint32_t		mtx_id;			/* 16-19 */
};

#if 0
#define	__HAVE_MUTEX_STUBS	1
#define	__HAVE_SPIN_MUTEX_STUBS	1
#endif

static inline uintptr_t
MUTEX_OWNER(uintptr_t owner)
{
	return owner;
}

static inline int
MUTEX_OWNED(uintptr_t owner)
{
	return owner != 0;
}

static inline int
MUTEX_SET_WAITERS(kmutex_t *mtx, uintptr_t owner)
{
	(void)__cpu_simple_lock_try(&mtx->mtx_lock);
 	return mtx->mtx_owner != 0;
}

static inline void
MUTEX_CLEAR_WAITERS(kmutex_t *mtx)
{
	__cpu_simple_unlock(&mtx->mtx_lock);
}

static inline int
MUTEX_HAS_WAITERS(volatile kmutex_t *mtx)
{
	if (mtx->mtx_owner == 0)
		return 0;
	return mtx->mtx_lock == __SIMPLELOCK_LOCKED;
}

static inline void
MUTEX_INITIALIZE_SPIN(kmutex_t *mtx, u_int id, int ipl)
{
	mtx->mtx_id = (id << 1) | 1;
	mtx->mtx_ipl = makeiplcookie(ipl);
	mtx->mtx_interlock = __SIMPLELOCK_LOCKED;
	__cpu_simple_lock_init(&mtx->mtx_lock);
}

static inline void
MUTEX_INITIALIZE_ADAPTIVE(kmutex_t *mtx, u_int id)
{
	mtx->mtx_id = (id << 1) | 0;
	__cpu_simple_lock_init(&mtx->mtx_interlock);
	__cpu_simple_lock_init(&mtx->mtx_lock);
}

static inline void
MUTEX_DESTROY(kmutex_t *mtx)
{
	mtx->mtx_owner = (uintptr_t)-1L;
	mtx->mtx_id = ~0;
}

static inline u_int
MUTEX_GETID(kmutex_t *mtx)
{
	return mtx->mtx_id >> 1;
}

static inline bool
MUTEX_SPIN_P(volatile kmutex_t *mtx)
{
	return (mtx->mtx_id & 1) == 1;
}

static inline bool
MUTEX_ADAPTIVE_P(volatile kmutex_t *mtx)
{
	return (mtx->mtx_id & 1) == 0;
}

static inline int
MUTEX_ACQUIRE(kmutex_t *mtx, uintptr_t curthread)
{
	if (!__cpu_simple_lock_try(&mtx->mtx_interlock))
		return 0;
	mtx->mtx_owner = curthread;
	return 1;
}

static inline void
MUTEX_RELEASE(kmutex_t *mtx)
{
	mtx->mtx_owner = 0;
	__cpu_simple_unlock(&mtx->mtx_lock);
	__cpu_simple_unlock(&mtx->mtx_interlock);
}

#endif	/* __MUTEX_PRIVATE */

#endif /* _ARM_MUTEX_H_ */