Function __rw_downgrade_int

freebsd/kern/kern_rwlock.c:1361–1423

* Downgrade a write lock into a single read lock. */

 * Downgrade a write lock into a single read lock.
 */
void
__rw_downgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
{
	struct turnstile *ts;
	uintptr_t tid, v;
	int rwait, wwait;

	if (SCHEDULER_STOPPED())
		return;

	KASSERT(rw->rw_lock != RW_DESTROYED,
	    ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
	__rw_assert(&rw->rw_lock, 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);
	} else
		turnstile_disown(ts);
	turnstile_chain_unlock(&rw->lock_object);