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/* $OpenBSD: kern_rwlock.c,v 1.6 2006/04/20 14:36:24 pedro 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>
/* 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 completly 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.
* RW_UPGRADE There must be exactly one holder of the read lock.
* We increment with what's needed for RW_WRITE - RW_READ.
* RWLOCK_WAIT|RWLOCK_WRWANT while waiting.
* RW_DOWNGRADE Always doable. Increment with -RW_WRITE + RW_READ.
*/
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_UPGRADE */
RWLOCK_WRLOCK-RWLOCK_READ_INCR,
~(RWLOCK_READ_INCR | RWLOCK_WAIT | RWLOCK_WRWANT),
RWLOCK_WAIT|RWLOCK_WRLOCK,
1,
PLOCK - 4
},
{ /* RW_DOWNGRADE */
-RWLOCK_WRLOCK + RWLOCK_READ_INCR,
0,
0,
-1,
0
}
};
#ifndef __HAVE_MD_RWLOCK
/*
* Simple cases that should be in MD code and atomic.
*/
void
rw_enter_read(struct rwlock *rwl)
{
if (__predict_false(rwl->rwl_owner & RWLOCK_WRLOCK))
rw_enter(rwl, RW_READ);
else
rwl->rwl_owner += RWLOCK_READ_INCR;
}
void
rw_enter_write(struct rwlock *rwl)
{
struct proc *p = curproc;
if (__predict_false(rwl->rwl_owner != 0))
rw_enter(rwl, RW_WRITE);
else
rwl->rwl_owner = RW_PROC(p) | RWLOCK_WRLOCK;
}
void
rw_exit_read(struct rwlock *rwl)
{
unsigned long owner = rwl->rwl_owner;
unsigned long decr = (owner & (RWLOCK_WAIT|RWLOCK_WRWANT)) |
RWLOCK_READ_INCR;
rwl->rwl_owner -= decr;
/*
* Potential MP race here. If the owner had WRWANT set we cleared
* it and a reader can sneak in before a writer. Do we care?
*/
if (__predict_false(owner & RWLOCK_WAIT))
rw_exit_waiters(rwl, owner);
}
void
rw_exit_write(struct rwlock *rwl)
{
unsigned long owner = rwl->rwl_owner;
rwl->rwl_owner = 0;
/*
* Potential MP race here. If the owner had WRWANT set we cleared
* it and a reader can sneak in before a writer. Do we care?
*/
if (__predict_false(owner & RWLOCK_WAIT))
rw_exit_waiters(rwl, owner);
}
int
rw_test_and_set(volatile unsigned long *p, unsigned long o, unsigned long n)
{
if (*p != o)
return (1);
*p = n;
return (0);
}
#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_UPGRADE:
/*
* Since we're holding the read lock, it can't possibly
* be write locked.
*/
if (rwl->rwl_owner & RWLOCK_WRLOCK)
panic("rw_enter: upgraded lock write locked");
break;
case RW_DOWNGRADE:
/*
* If we're downgrading, we must hold the write lock.
*/
if (RW_PROC(curproc) != RW_PROC(rwl->rwl_owner))
panic("rw_enter: not holder");
default:
panic("rw_enter: unknown op 0x%x", flags);
}
}
static void
rw_exit_diag(struct rwlock *rwl, int owner)
{
if ((owner & RWLOCK_WAIT) == 0)
panic("rw_exit: no waiter");
}
#else
#define rw_enter_diag(r, f)
#define rw_exit_diag(r, o)
#endif
void
rw_init(struct rwlock *rwl)
{
rwl->rwl_owner = 0;
}
/*
* You are supposed to understand this.
*/
int
rw_enter(struct rwlock *rwl, int flags)
{
const struct rwlock_op *op;
unsigned long inc, o;
int error, prio;
op = &rw_ops[flags & RW_OPMASK];
inc = op->inc;
if (op->proc_mult == -1)
inc -= RW_PROC(curproc);
else if (op->proc_mult == 1)
inc += RW_PROC(curproc);
prio = op->wait_prio;
if (flags & RW_INTR)
prio |= PCATCH;
retry:
while (__predict_false(((o = rwl->rwl_owner) & op->check) != 0)) {
if (rw_test_and_set(&rwl->rwl_owner, o, o | op->wait_set))
continue;
rw_enter_diag(rwl, flags);
if ((error = tsleep(rwl, prio, "rwlock", 0)) != 0)
return (error);
if (flags & RW_SLEEPFAIL)
return (EAGAIN);
}
if (__predict_false(rw_test_and_set(&rwl->rwl_owner, o, o + inc)))
goto retry;
return (0);
}
void
rw_exit_waiters(struct rwlock *rwl, unsigned long owner)
{
rw_exit_diag(rwl, owner);
/* We wake up all waiters because we can't know how many they are. */
wakeup(rwl);
}
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