Fix the turnstile_lock() KPI.

turnstile_{lock,unlock}() were added for use in epoch.  turnstile_lock()
returned NULL to indicate that the calling thread had lost a race and
the turnstile was no longer associated with the given lock, or the lock
owner.  However, reader-writer locks may not have a designated owner,
in which case turnstile_lock() would return NULL and
epoch_block_handler_preempt() would leak spinlocks as a result.

Apply a minimal fix: return the lock owner as a separate return value.

Reviewed by:	kib
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D21048

sys/kern/subr_epoch.c

@@ -438,26 +438,20 @@ epoch_block_handler_preempt(struct ck_epoch *global __unused,
 			 */
 			critical_enter();
 			thread_unlock(td);
-			owner = turnstile_lock(ts, &lock);
+
-			/*
+			if (turnstile_lock(ts, &lock, &owner)) {
-			 * The owner pointer indicates that the lock succeeded.
+				if (ts == curwaittd->td_blocked) {
-			 * Only in case we hold the lock and the turnstile we
+					MPASS(TD_IS_INHIBITED(curwaittd) &&
-			 * locked is still the one that curwaittd is blocked on
+					    TD_ON_LOCK(curwaittd));
-			 * can we continue. Otherwise the turnstile pointer has
+					critical_exit();
-			 * been changed out from underneath us, as in the case
+					turnstile_wait(ts, owner,
-			 * where the lock holder has signalled curwaittd,
+					    curwaittd->td_tsqueue);
-			 * and we need to continue.
+					counter_u64_add(turnstile_count, 1);
-			 */
+					thread_lock(td);
-			if (owner != NULL && ts == curwaittd->td_blocked) {
+					return;
-				MPASS(TD_IS_INHIBITED(curwaittd) &&
+				}
-				    TD_ON_LOCK(curwaittd));
-				critical_exit();
-				turnstile_wait(ts, owner, curwaittd->td_tsqueue);
-				counter_u64_add(turnstile_count, 1);
-				thread_lock(td);
-				return;
-			} else if (owner != NULL)
 				turnstile_unlock(ts, lock);
+			}
 			thread_lock(td);
 			critical_exit();
 			KASSERT(td->td_locks == locksheld,

sys/kern/subr_turnstile.c

@@ -566,24 +566,26 @@ turnstile_trywait(struct lock_object *lock)
 	return (ts);
 }
 
-struct thread *
+bool
-turnstile_lock(struct turnstile *ts, struct lock_object **lockp)
+turnstile_lock(struct turnstile *ts, struct lock_object **lockp,
+    struct thread **tdp)
 {
 	struct turnstile_chain *tc;
 	struct lock_object *lock;
 
 	if ((lock = ts->ts_lockobj) == NULL)
-		return (NULL);
+		return (false);
 	tc = TC_LOOKUP(lock);
 	mtx_lock_spin(&tc->tc_lock);
 	mtx_lock_spin(&ts->ts_lock);
 	if (__predict_false(lock != ts->ts_lockobj)) {
 		mtx_unlock_spin(&tc->tc_lock);
 		mtx_unlock_spin(&ts->ts_lock);
-		return (NULL);
+		return (false);
 	}
 	*lockp = lock;
-	return (ts->ts_owner);
+	*tdp = ts->ts_owner;
+	return (true);
 }
 
 void

sys/sys/turnstile.h

@@ -99,7 +99,8 @@ int	turnstile_signal(struct turnstile *, int);
 struct turnstile *turnstile_trywait(struct lock_object *);
 void	turnstile_unpend(struct turnstile *);
 void	turnstile_wait(struct turnstile *, struct thread *, int);
-struct thread *turnstile_lock(struct turnstile *, struct lock_object **);
+bool	turnstile_lock(struct turnstile *, struct lock_object **,
+	    struct thread **);
 void	turnstile_unlock(struct turnstile *, struct lock_object *);
 void	turnstile_assert(struct turnstile *);
 #endif	/* _KERNEL */