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/* $OpenBSD: kern_rwlock.c,v 1.12 2007/05/04 13:21:03 art Exp $ */
/*
* Copyright (c) 2002, 2003 Artur Grabowski <art@openbsd.org>
* 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. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED ``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 AUTHOR 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/rwlock.h>
#include <sys/limits.h>
#include <machine/lock.h>
/* XXX - temporary measure until proc0 is properly aligned */
#define RW_PROC(p) (((long)p) & ~RWLOCK_MASK)
/*
* Magic wand for lock operations. Every operation checks if certain
* flags are set and if they aren't, it increments the lock with some
* value (that might need some computing in a few cases). If the operation
* fails, we need to set certain flags while waiting for the lock.
*
* RW_WRITE The lock must be completely empty. We increment it with
* RWLOCK_WRLOCK and the proc pointer of the holder.
* Sets RWLOCK_WAIT|RWLOCK_WRWANT while waiting.
* RW_READ RWLOCK_WRLOCK|RWLOCK_WRWANT may not be set. We increment
* with RWLOCK_READ_INCR. RWLOCK_WAIT while waiting.
*/
static const struct rwlock_op {
unsigned long inc;
unsigned long check;
unsigned long wait_set;
long proc_mult;
int wait_prio;
} rw_ops[] = {
{ /* RW_WRITE */
RWLOCK_WRLOCK,
ULONG_MAX,
RWLOCK_WAIT | RWLOCK_WRWANT,
1,
PLOCK - 4
},
{ /* RW_READ */
RWLOCK_READ_INCR,
RWLOCK_WRLOCK,
RWLOCK_WAIT,
0,
PLOCK
},
{ /* RW_DOWNGRADE */
RWLOCK_READ_INCR - RWLOCK_WRLOCK,
0,
0,
-1,
PLOCK
},
};
#ifndef __HAVE_MD_RWLOCK
/*
* Simple cases that should be in MD code and atomic.
*/
void
rw_enter_read(struct rwlock *rwl)
{
unsigned long owner = rwl->rwl_owner;
if (__predict_false((owner & RWLOCK_WRLOCK) ||
rw_cas(&rwl->rwl_owner, owner, owner + RWLOCK_READ_INCR)))
rw_enter(rwl, RW_READ);
}
void
rw_enter_write(struct rwlock *rwl)
{
struct proc *p = curproc;
if (__predict_false(rw_cas(&rwl->rwl_owner, 0,
RW_PROC(p) | RWLOCK_WRLOCK)))
rw_enter(rwl, RW_WRITE);
}
void
rw_exit_read(struct rwlock *rwl)
{
unsigned long owner = rwl->rwl_owner;
if (__predict_false((owner & RWLOCK_WAIT) ||
rw_cas(&rwl->rwl_owner, owner, owner - RWLOCK_READ_INCR)))
rw_exit(rwl);
}
void
rw_exit_write(struct rwlock *rwl)
{
unsigned long owner = rwl->rwl_owner;
if (__predict_false((owner & RWLOCK_WAIT) ||
rw_cas(&rwl->rwl_owner, owner, 0)))
rw_exit(rwl);
}
#ifndef rw_cas
int
rw_cas(volatile unsigned long *p, unsigned long o, unsigned long n)
{
if (*p != o)
return (1);
*p = n;
return (0);
}
#endif
#endif
#ifdef DIAGNOSTIC
/*
* Put the diagnostic functions here to keep the main code free
* from ifdef clutter.
*/
static void
rw_enter_diag(struct rwlock *rwl, int flags)
{
switch (flags & RW_OPMASK) {
case RW_WRITE:
case RW_READ:
if (RW_PROC(curproc) == RW_PROC(rwl->rwl_owner))
panic("rw_enter: locking against myself");
break;
case RW_DOWNGRADE:
/*
* If we're downgrading, we must hold the write lock.
*/
if ((rwl->rwl_owner & RWLOCK_WRLOCK) == 0)
panic("rw_enter: downgrade of non-write lock");
if (RW_PROC(curproc) != RW_PROC(rwl->rwl_owner))
panic("rw_enter: downgrade, not holder");
break;
default:
panic("rw_enter: unknown op 0x%x", flags);
}
}
#else
#define rw_enter_diag(r, f)
#endif
void
rw_init(struct rwlock *rwl, const char *name)
{
rwl->rwl_owner = 0;
rwl->rwl_name = name;
}
int
rw_enter(struct rwlock *rwl, int flags)
{
const struct rwlock_op *op;
struct sleep_state sls;
unsigned long inc, o;
int error;
op = &rw_ops[flags & RW_OPMASK];
inc = op->inc + RW_PROC(curproc) * op->proc_mult;
retry:
while (__predict_false(((o = rwl->rwl_owner) & op->check) != 0)) {
unsigned long set = o | op->wait_set;
int do_sleep;
rw_enter_diag(rwl, flags);
if (flags & RW_NOSLEEP)
return (EBUSY);
sleep_setup(&sls, rwl, op->wait_prio, rwl->rwl_name);
if (flags & RW_INTR)
sleep_setup_signal(&sls, op->wait_prio | PCATCH);
do_sleep = !rw_cas(&rwl->rwl_owner, o, set);
sleep_finish(&sls, do_sleep);
if ((flags & RW_INTR) &&
(error = sleep_finish_signal(&sls)) != 0)
return (error);
if (flags & RW_SLEEPFAIL)
return (EAGAIN);
}
if (__predict_false(rw_cas(&rwl->rwl_owner, o, o + inc)))
goto retry;
/*
* If old lock had RWLOCK_WAIT and RWLOCK_WRLOCK set, it means we
* downgraded a write lock and had possible read waiter, wake them
* to let them retry the lock.
*/
if (__predict_false((o & (RWLOCK_WRLOCK|RWLOCK_WAIT)) ==
(RWLOCK_WRLOCK|RWLOCK_WAIT)))
wakeup(rwl);
return (0);
}
void
rw_exit(struct rwlock *rwl)
{
unsigned long owner = rwl->rwl_owner;
int wrlock = owner & RWLOCK_WRLOCK;
unsigned long set;
do {
owner = rwl->rwl_owner;
if (wrlock)
set = 0;
else
set = (owner - RWLOCK_READ_INCR) &
~(RWLOCK_WAIT|RWLOCK_WRWANT);
} while (rw_cas(&rwl->rwl_owner, owner, set));
if (owner & RWLOCK_WAIT)
wakeup(rwl);
}
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