/*
* 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;
if (SCHEDULER_STOPPED())
return;
if (rw_wlocked(rw) && rw_recursed(rw)) {
rw->rw_recurse--;
if (LOCK_LOG_TEST(&rw->lock_object, 0))
CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
return;
}
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->lock_object, 0))
CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
turnstile_chain_lock(&rw->lock_object);
ts = turnstile_lookup(&rw->lock_object);
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.
*/
v = RW_UNLOCKED;
if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
queue = TS_EXCLUSIVE_QUEUE;
v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
} else
queue = TS_SHARED_QUEUE;
/* Wake up all waiters for the specific queue. */
if (LOCK_LOG_TEST(&rw->lock_object, 0))
CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
queue == TS_SHARED_QUEUE ? "read" : "write");
turnstile_broadcast(ts, queue);
atomic_store_rel_ptr(&rw->rw_lock, v);
turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
turnstile_chain_unlock(&rw->lock_object);
}
void
_rw_wunlock(struct rwlock *rw, const char *file, int line)
{
if (SCHEDULER_STOPPED())
return;
KASSERT(rw->rw_lock != RW_DESTROYED,
("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
_rw_assert(rw, RA_WLOCKED, file, line);
curthread->td_locks--;
WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
line);
if (!rw_recursed(rw))
LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw);
__rw_wunlock(rw, curthread, file, line);
}
void
db_show_rwlock(const struct lock_object *lock)
{
const struct rwlock *rw;
struct thread *td;
rw = (const struct rwlock *)lock;
db_printf(" state: ");
if (rw->rw_lock == RW_UNLOCKED)
db_printf("UNLOCKED\n");
else if (rw->rw_lock == RW_DESTROYED) {
db_printf("DESTROYED\n");
return;
} else if (rw->rw_lock & RW_LOCK_READ)
db_printf("RLOCK: %ju locks\n",
(uintmax_t)(RW_READERS(rw->rw_lock)));
else {
td = rw_wowner(rw);
db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
td->td_tid, td->td_proc->p_pid, td->td_name);
if (rw_recursed(rw))
db_printf(" recursed: %u\n", rw->rw_recurse);
}
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 writers\n");
break;
default:
db_printf("none\n");
break;
}
}
int
__rw_try_wlock(volatile uintptr_t *c, const char *file, int line)
{
struct rwlock *rw;
int rval;
if (SCHEDULER_STOPPED())
return (1);
rw = rwlock2rw(c);
KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d",
curthread, rw->lock_object.lo_name, file, line));
KASSERT(rw->rw_lock != RW_DESTROYED,
("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
if (rw_wlocked(rw) &&
(rw->lock_object.lo_flags & LO_RECURSABLE) != 0) {
rw->rw_recurse++;
rval = 1;
} else
rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED,
(uintptr_t)curthread);
LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
if (rval) {
WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
file, line);
if (!rw_recursed(rw))
LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
rw, 0, 0, file, line, LOCKSTAT_WRITER);
curthread->td_locks++;
}
return (rval);
}
/*
* 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(const volatile uintptr_t *c, int what, const char *file, int line)
{
const struct rwlock *rw;
if (panicstr != NULL)
return;
rw = rwlock2rw(c);
switch (what) {
case RA_LOCKED:
case RA_LOCKED | RA_RECURSED:
case RA_LOCKED | RA_NOTRECURSED:
case RA_RLOCKED:
case RA_RLOCKED | RA_RECURSED:
case RA_RLOCKED | RA_NOTRECURSED:
#ifdef WITNESS
witness_assert(&rw->lock_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_wowner(rw) != curthread)))
panic("Lock %s not %slocked @ %s:%d\n",
rw->lock_object.lo_name, (what & RA_RLOCKED) ?
"read " : "", file, line);
if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
if (rw_recursed(rw)) {
if (what & RA_NOTRECURSED)
panic("Lock %s recursed @ %s:%d\n",
rw->lock_object.lo_name, file,
line);
} else if (what & RA_RECURSED)
panic("Lock %s not recursed @ %s:%d\n",
rw->lock_object.lo_name, file, line);
}
#endif
break;
case RA_WLOCKED:
case RA_WLOCKED | RA_RECURSED:
case RA_WLOCKED | RA_NOTRECURSED:
if (rw_wowner(rw) != curthread)
panic("Lock %s not exclusively locked @ %s:%d\n",
rw->lock_object.lo_name, file, line);
if (rw_recursed(rw)) {
if (what & RA_NOTRECURSED)
panic("Lock %s recursed @ %s:%d\n",
rw->lock_object.lo_name, file, line);
} else if (what & RA_RECURSED)
panic("Lock %s not recursed @ %s:%d\n",
rw->lock_object.lo_name, file, line);
break;
case RA_UNLOCKED:
#ifdef WITNESS
witness_assert(&rw->lock_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_wowner(rw) == curthread)
panic("Lock %s exclusively locked @ %s:%d\n",
rw->lock_object.lo_name, file, line);
#endif
break;
default:
panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
line);
}
}
/*
* Downgrade a write lock into a single read lock.
*/
void
__rw_downgrade(volatile uintptr_t *c, const char *file, int line)
{
struct rwlock *rw;
struct turnstile *ts;
uintptr_t tid, v;
int rwait, wwait;
if (SCHEDULER_STOPPED())
return;
rw = rwlock2rw(c);
KASSERT(rw->rw_lock != RW_DESTROYED,
("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
__rw_assert(c, RA_WLOCKED | RA_NOTRECURSED, file, line);
#ifndef INVARIANTS
if (rw_recursed(rw))
panic("downgrade of a recursed lock");
#endif
WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
/*
* Convert from a writer to a single reader. First we handle
* the easy case with no waiters. If there are any waiters, we
* lock the turnstile and "disown" the lock.
*/
tid = (uintptr_t)curthread;
if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
goto out;
/*
* Ok, we think we have waiters, so lock the turnstile so we can
* read the waiter flags without any races.
*/
turnstile_chain_lock(&rw->lock_object);
v = rw->rw_lock & RW_LOCK_WAITERS;
rwait = v & RW_LOCK_READ_WAITERS;
wwait = v & RW_LOCK_WRITE_WAITERS;
MPASS(rwait | wwait);
/*
* Downgrade from a write lock while preserving waiters flag
* and give up ownership of the turnstile.
*/
ts = turnstile_lookup(&rw->lock_object);
MPASS(ts != NULL);
if (!wwait)
v &= ~RW_LOCK_READ_WAITERS;
atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
/*
* Wake other readers if there are no writers pending. Otherwise they
* won't be able to acquire the lock anyway.
*/
if (rwait && !wwait) {
turnstile_broadcast(ts, TS_SHARED_QUEUE);
turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
} else
turnstile_disown(ts);
turnstile_chain_unlock(&rw->lock_object);
out:
curthread->td_rw_rlocks++;
LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
LOCKSTAT_RECORD0(rw__downgrade, rw);
}
