/*
* Downgrade an unrecursed exclusive lock into a single shared lock.
*/
void
_sx_downgrade(struct sx *sx, const char *file, int line)
{
uintptr_t x;
int wakeup_swapper;
KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
("sx_downgrade() of destroyed sx @ %s:%d", file, line));
_sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
#ifndef INVARIANTS
if (sx_recursed(sx))
panic("downgrade of a recursed lock");
#endif
WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
/*
* Try to switch from an exclusive lock with no shared waiters
* to one sharer with no shared waiters. If there are
* exclusive waiters, we don't need to lock the sleep queue so
* long as we preserve the flag. We do one quick try and if
* that fails we grab the sleepq lock to keep the flags from
* changing and do it the slow way.
*
* We have to lock the sleep queue if there are shared waiters
* so we can wake them up.
*/
x = sx->sx_lock;
if (!(x & SX_LOCK_SHARED_WAITERS) &&
atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
(x & SX_LOCK_EXCLUSIVE_WAITERS))) {
LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
return;
}
/*
* Lock the sleep queue so we can read the waiters bits
* without any races and wakeup any shared waiters.
*/
sleepq_lock(&sx->lock_object);
/*
* Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
* shared lock. If there are any shared waiters, wake them up.
*/
wakeup_swapper = 0;
x = sx->sx_lock;
atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
(x & SX_LOCK_EXCLUSIVE_WAITERS));
if (x & SX_LOCK_SHARED_WAITERS)
wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
0, SQ_SHARED_QUEUE);
sleepq_release(&sx->lock_object);
LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
LOCKSTAT_RECORD0(LS_SX_DOWNGRADE_DOWNGRADE, sx);
if (wakeup_swapper)
kick_proc0();
}
void
_sx_downgrade(struct sx *sx, const char *file, int line)
{
_sx_assert(sx, SX_XLOCKED, file, line);
mtx_lock(sx->sx_lock);
MPASS(sx->sx_cnt == -1);
WITNESS_DOWNGRADE(&sx->sx_object, 0, file, line);
sx->sx_cnt = 1;
sx->sx_xholder = NULL;
if (sx->sx_shrd_wcnt > 0)
cv_broadcast(&sx->sx_shrd_cv);
LOCK_LOG_LOCK("XDOWNGRADE", &sx->sx_object, 0, 0, file, line);
mtx_unlock(sx->sx_lock);
}
/*
* 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);
}