freebsd-dev/sys/kern/kern_rwlock.c
2006-02-01 04:18:07 +00:00

589 lines
17 KiB
C

/*-
* Copyright (c) 2006 John Baldwin <jhb@FreeBSD.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. 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 author nor the names of any co-contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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.
*/
/*
* Machine independent bits of reader/writer lock implementation.
*/
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include "opt_ddb.h"
#include <sys/param.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/rwlock.h>
#include <sys/systm.h>
#include <sys/turnstile.h>
#include <machine/cpu.h>
#ifdef DDB
#include <ddb/ddb.h>
static void db_show_rwlock(struct lock_object *lock);
#endif
struct lock_class lock_class_rw = {
"rw",
LC_SLEEPLOCK | LC_RECURSABLE /* | LC_UPGRADABLE */,
#ifdef DDB
db_show_rwlock
#endif
};
#define rw_owner(rw) \
((rw)->rw_lock & RW_LOCK_READ ? NULL : \
(struct thread *)RW_OWNER((rw)->rw_lock))
#ifndef INVARIANTS
#define _rw_assert(rw, what, file, line)
#endif
void
rw_init(struct rwlock *rw, const char *name)
{
rw->rw_lock = RW_UNLOCKED;
lock_init(&rw->rw_object, &lock_class_rw, name, NULL, LO_WITNESS |
LO_RECURSABLE /* | LO_UPGRADABLE */);
}
void
rw_destroy(struct rwlock *rw)
{
KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock not unlocked"));
lock_destroy(&rw->rw_object);
}
void
rw_sysinit(void *arg)
{
struct rw_args *args = arg;
rw_init(args->ra_rw, args->ra_desc);
}
void
_rw_wlock(struct rwlock *rw, const char *file, int line)
{
MPASS(curthread != NULL);
KASSERT(rw_owner(rw) != curthread,
("%s (%s): wlock already held @ %s:%d", __func__,
rw->rw_object.lo_name, file, line));
WITNESS_CHECKORDER(&rw->rw_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
line);
__rw_wlock(rw, curthread, file, line);
LOCK_LOG_LOCK("WLOCK", &rw->rw_object, 0, 0, file, line);
WITNESS_LOCK(&rw->rw_object, LOP_EXCLUSIVE, file, line);
}
void
_rw_wunlock(struct rwlock *rw, const char *file, int line)
{
MPASS(curthread != NULL);
_rw_assert(rw, RA_WLOCKED, file, line);
WITNESS_UNLOCK(&rw->rw_object, LOP_EXCLUSIVE, file, line);
LOCK_LOG_LOCK("WUNLOCK", &rw->rw_object, 0, 0, file, line);
__rw_wunlock(rw, curthread, file, line);
}
void
_rw_rlock(struct rwlock *rw, const char *file, int line)
{
uintptr_t x;
KASSERT(rw_owner(rw) != curthread,
("%s (%s): wlock already held @ %s:%d", __func__,
rw->rw_object.lo_name, file, line));
WITNESS_CHECKORDER(&rw->rw_object, LOP_NEWORDER, file, line);
/*
* Note that we don't make any attempt to try to block read
* locks once a writer has blocked on the lock. The reason is
* that we currently allow for read locks to recurse and we
* don't keep track of all the holders of read locks. Thus, if
* we were to block readers once a writer blocked and a reader
* tried to recurse on their reader lock after a writer had
* blocked we would end up in a deadlock since the reader would
* be blocked on the writer, and the writer would be blocked
* waiting for the reader to release its original read lock.
*/
for (;;) {
/*
* Handle the easy case. If no other thread has a write
* lock, then try to bump up the count of read locks. Note
* that we have to preserve the current state of the
* RW_LOCK_WRITE_WAITERS flag. If we fail to acquire a
* read lock, then rw_lock must have changed, so restart
* the loop. Note that this handles the case of a
* completely unlocked rwlock since such a lock is encoded
* as a read lock with no waiters.
*/
x = rw->rw_lock;
if (x & RW_LOCK_READ) {
/*
* The RW_LOCK_READ_WAITERS flag should only be set
* if another thread currently holds a write lock,
* and in that case RW_LOCK_READ should be clear.
*/
MPASS((x & RW_LOCK_READ_WAITERS) == 0);
if (atomic_cmpset_acq_ptr(&rw->rw_lock, x,
x + RW_ONE_READER)) {
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR4(KTR_LOCK,
"%s: %p succeed %p -> %p", __func__,
rw, (void *)x,
(void *)(x + RW_ONE_READER));
break;
}
continue;
}
/*
* Okay, now it's the hard case. Some other thread already
* has a write lock, so acquire the turnstile lock so we can
* begin the process of blocking.
*/
turnstile_lock(&rw->rw_object);
/*
* The lock might have been released while we spun, so
* recheck its state and restart the loop if there is no
* longer a write lock.
*/
x = rw->rw_lock;
if (x & RW_LOCK_READ) {
turnstile_release(&rw->rw_object);
continue;
}
/*
* Ok, it's still a write lock. If the RW_LOCK_READ_WAITERS
* flag is already set, then we can go ahead and block. If
* it is not set then try to set it. If we fail to set it
* drop the turnstile lock and restart the loop.
*/
if (!(x & RW_LOCK_READ_WAITERS) &&
!atomic_cmpset_ptr(&rw->rw_lock, x,
x | RW_LOCK_READ_WAITERS)) {
turnstile_release(&rw->rw_object);
continue;
}
if (!(x & RW_LOCK_READ_WAITERS) &&
LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p set read waiters flag", __func__,
rw);
/*
* We were unable to acquire the lock and the read waiters
* flag is set, so we must block on the turnstile.
*/
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
rw);
turnstile_wait(&rw->rw_object, rw_owner(rw), TS_SHARED_QUEUE);
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
__func__, rw);
}
/*
* TODO: acquire "owner of record" here. Here be turnstile dragons
* however. turnstiles don't like owners changing between calls to
* turnstile_wait() currently.
*/
LOCK_LOG_LOCK("RLOCK", &rw->rw_object, 0, 0, file, line);
WITNESS_LOCK(&rw->rw_object, 0, file, line);
}
void
_rw_runlock(struct rwlock *rw, const char *file, int line)
{
struct turnstile *ts;
uintptr_t x;
_rw_assert(rw, RA_RLOCKED, file, line);
WITNESS_UNLOCK(&rw->rw_object, 0, file, line);
LOCK_LOG_LOCK("RUNLOCK", &rw->rw_object, 0, 0, file, line);
/* TODO: drop "owner of record" here. */
for (;;) {
/*
* See if there is more than one read lock held. If so,
* just drop one and return.
*/
x = rw->rw_lock;
if (RW_READERS(x) > 1) {
if (atomic_cmpset_ptr(&rw->rw_lock, x,
x - RW_ONE_READER)) {
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR4(KTR_LOCK,
"%s: %p succeeded %p -> %p",
__func__, rw, (void *)x,
(void *)(x - RW_ONE_READER));
break;
}
continue;
}
/*
* We should never have read waiters while at least one
* thread holds a read lock. (See note above)
*/
KASSERT(!(x & RW_LOCK_READ_WAITERS),
("%s: waiting readers", __func__));
/*
* If there aren't any waiters for a write lock, then try
* to drop it quickly.
*/
if (!(x & RW_LOCK_WRITE_WAITERS)) {
/*
* There shouldn't be any flags set and we should
* be the only read lock. If we fail to release
* the single read lock, then another thread might
* have just acquired a read lock, so go back up
* to the multiple read locks case.
*/
MPASS(x == RW_READERS_LOCK(1));
if (atomic_cmpset_ptr(&rw->rw_lock, RW_READERS_LOCK(1),
RW_UNLOCKED)) {
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p last succeeded",
__func__, rw);
break;
}
continue;
}
/*
* There should just be one reader with one or more
* writers waiting.
*/
MPASS(x == (RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS));
/*
* Ok, we know we have a waiting writer and we think we
* are the last reader, so grab the turnstile lock.
*/
turnstile_lock(&rw->rw_object);
/*
* Try to drop our lock leaving the lock in a unlocked
* state.
*
* If you wanted to do explicit lock handoff you'd have to
* do it here. You'd also want to use turnstile_signal()
* and you'd have to handle the race where a higher
* priority thread blocks on the write lock before the
* thread you wakeup actually runs and have the new thread
* "steal" the lock. For now it's a lot simpler to just
* wakeup all of the waiters.
*
* As above, if we fail, then another thread might have
* acquired a read lock, so drop the turnstile lock and
* restart.
*/
if (!atomic_cmpset_ptr(&rw->rw_lock,
RW_READERS_LOCK(1) | RW_LOCK_WRITE_WAITERS, RW_UNLOCKED)) {
turnstile_release(&rw->rw_object);
continue;
}
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
__func__, rw);
/*
* Ok. The lock is released and all that's left is to
* wake up the waiters. Note that the lock might not be
* free anymore, but in that case the writers will just
* block again if they run before the new lock holder(s)
* release the lock.
*/
ts = turnstile_lookup(&rw->rw_object);
turnstile_broadcast(ts, TS_EXCLUSIVE_QUEUE);
turnstile_unpend(ts, TS_SHARED_LOCK);
break;
}
}
/*
* This function is called when we are unable to obtain a write lock on the
* first try. This means that at least one other thread holds either a
* read or write lock.
*/
void
_rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
{
uintptr_t v;
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
rw->rw_object.lo_name, (void *)rw->rw_lock, file, line);
while (!_rw_write_lock(rw, tid)) {
turnstile_lock(&rw->rw_object);
v = rw->rw_lock;
/*
* If the lock was released while spinning on the
* turnstile chain lock, try again.
*/
if (v == RW_UNLOCKED) {
turnstile_release(&rw->rw_object);
cpu_spinwait();
continue;
}
/*
* If the lock was released by a writer with both readers
* and writers waiting and a reader hasn't woken up and
* acquired the lock yet, rw_lock will be set to the
* value RW_UNLOCKED | RW_LOCK_WRITE_WAITERS. If we see
* that value, try to acquire it once. Note that we have
* to preserve the RW_LOCK_WRITE_WAITERS flag as there are
* other writers waiting still. If we fail, restart the
* loop.
*/
if (v == (RW_UNLOCKED | RW_LOCK_WRITE_WAITERS)) {
if (atomic_cmpset_acq_ptr(&rw->rw_lock,
RW_UNLOCKED | RW_LOCK_WRITE_WAITERS,
tid | RW_LOCK_WRITE_WAITERS)) {
turnstile_claim(&rw->rw_object);
CTR2(KTR_LOCK, "%s: %p claimed by new writer",
__func__, rw);
break;
}
turnstile_release(&rw->rw_object);
cpu_spinwait();
continue;
}
/*
* If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
* set it. If we fail to set it, then loop back and try
* again.
*/
if (!(v & RW_LOCK_WRITE_WAITERS) &&
!atomic_cmpset_ptr(&rw->rw_lock, v,
v | RW_LOCK_WRITE_WAITERS)) {
turnstile_release(&rw->rw_object);
cpu_spinwait();
continue;
}
if (!(v & RW_LOCK_WRITE_WAITERS) &&
LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p set write waiters flag",
__func__, rw);
/* XXX: Adaptively spin if current wlock owner on another CPU? */
/*
* We were unable to acquire the lock and the write waiters
* flag is set, so we must block on the turnstile.
*/
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
rw);
turnstile_wait(&rw->rw_object, rw_owner(rw),
TS_EXCLUSIVE_QUEUE);
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
__func__, rw);
}
}
/*
* This function is called if the first try at releasing a write lock failed.
* This means that one of the 2 waiter bits must be set indicating that at
* least one thread is waiting on this lock.
*/
void
_rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
{
struct turnstile *ts;
uintptr_t v;
int queue;
KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
("%s: neither of the waiter flags are set", __func__));
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
turnstile_lock(&rw->rw_object);
ts = turnstile_lookup(&rw->rw_object);
/* XXX: Adaptive fixup would be required here. */
MPASS(ts != NULL);
/*
* Use the same algo as sx locks for now. Prefer waking up shared
* waiters if we have any over writers. This is probably not ideal.
*
* 'v' is the value we are going to write back to rw_lock. If we
* have waiters on both queues, we need to preserve the state of
* the waiter flag for the queue we don't wake up. For now this is
* hardcoded for the algorithm mentioned above.
*
* In the case of both readers and writers waiting we wakeup the
* readers but leave the RW_LOCK_WRITE_WAITERS flag set. If a
* new writer comes in before a reader it will claim the lock up
* above. There is probably a potential priority inversion in
* there that could be worked around either by waking both queues
* of waiters or doing some complicated lock handoff gymnastics.
*/
if (rw->rw_lock & RW_LOCK_READ_WAITERS) {
queue = TS_SHARED_QUEUE;
v = RW_UNLOCKED | (rw->rw_lock & RW_LOCK_WRITE_WAITERS);
} else {
queue = TS_EXCLUSIVE_QUEUE;
v = RW_UNLOCKED;
}
if (LOCK_LOG_TEST(&rw->rw_object, 0))
CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
queue == TS_SHARED_QUEUE ? "read" : "write");
/* Wake up all waiters for the specific queue. */
turnstile_broadcast(ts, queue);
atomic_store_rel_ptr(&rw->rw_lock, v);
turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
}
#ifdef INVARIANT_SUPPORT
#ifndef INVARIANTS
#undef _rw_assert
#endif
/*
* In the non-WITNESS case, rw_assert() can only detect that at least
* *some* thread owns an rlock, but it cannot guarantee that *this*
* thread owns an rlock.
*/
void
_rw_assert(struct rwlock *rw, int what, const char *file, int line)
{
if (panicstr != NULL)
return;
switch (what) {
case RA_LOCKED:
case RA_RLOCKED:
#ifdef WITNESS
witness_assert(&rw->rw_object, what, file, line);
#else
/*
* If some other thread has a write lock or we have one
* and are asserting a read lock, fail. Also, if no one
* has a lock at all, fail.
*/
if (rw->rw_lock == RW_UNLOCKED ||
(!(rw->rw_lock & RW_LOCK_READ) && (what == RA_RLOCKED ||
rw_owner(rw) != curthread)))
panic("Lock %s not %slocked @ %s:%d\n",
rw->rw_object.lo_name, (what == RA_RLOCKED) ?
"read " : "", file, line);
#endif
break;
case RA_WLOCKED:
if (rw_owner(rw) != curthread)
panic("Lock %s not exclusively locked @ %s:%d\n",
rw->rw_object.lo_name, file, line);
break;
case RA_UNLOCKED:
#ifdef WITNESS
witness_assert(&rw->rw_object, what, file, line);
#else
/*
* If we hold a write lock fail. We can't reliably check
* to see if we hold a read lock or not.
*/
if (rw_owner(rw) == curthread)
panic("Lock %s exclusively locked @ %s:%d\n",
rw->rw_object.lo_name, file, line);
#endif
break;
default:
panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
line);
}
}
#endif /* INVARIANT_SUPPORT */
#ifdef DDB
void
db_show_rwlock(struct lock_object *lock)
{
struct rwlock *rw;
struct thread *td;
rw = (struct rwlock *)lock;
db_printf(" state: ");
if (rw->rw_lock == RW_UNLOCKED)
db_printf("UNLOCKED\n");
else if (rw->rw_lock & RW_LOCK_READ)
db_printf("RLOCK: %jd locks\n",
(intmax_t)(RW_READERS(rw->rw_lock)));
else {
td = rw_owner(rw);
db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm);
}
db_printf(" waiters: ");
switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
case RW_LOCK_READ_WAITERS:
db_printf("readers\n");
break;
case RW_LOCK_WRITE_WAITERS:
db_printf("writers\n");
break;
case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
db_printf("readers and waiters\n");
break;
default:
db_printf("none\n");
break;
}
}
#endif