svn commit: r350310 - in head/sys: kern sys

[Mark Johnston]

Author: markj
Date: Wed Jul 24 23:04:59 2019
New Revision: 350310
URL: https://svnweb.freebsd.org/changeset/base/350310

Log:
  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

Modified:
  head/sys/kern/subr_epoch.c
  head/sys/kern/subr_turnstile.c
  head/sys/sys/turnstile.h

Modified: head/sys/kern/subr_epoch.c
==============================================================================
--- head/sys/kern/subr_epoch.c	Wed Jul 24 23:04:26 2019	(r350309)
+++ head/sys/kern/subr_epoch.c	Wed Jul 24 23:04:59 2019	(r350310)
@@ -438,26 +438,20 @@ epoch_block_handler_preempt(struct ck_epoch *global __
 			 */
 			critical_enter();
 			thread_unlock(td);
-			owner = turnstile_lock(ts, &lock);
-			/*
-			 * The owner pointer indicates that the lock succeeded.
-			 * Only in case we hold the lock and the turnstile we
-			 * locked is still the one that curwaittd is blocked on
-			 * can we continue. Otherwise the turnstile pointer has
-			 * been changed out from underneath us, as in the case
-			 * where the lock holder has signalled curwaittd,
-			 * and we need to continue.
-			 */
-			if (owner != NULL && ts == curwaittd->td_blocked) {
-				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)
+
+			if (turnstile_lock(ts, &lock, &owner)) {
+				if (ts == curwaittd->td_blocked) {
+					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;
+				}
 				turnstile_unlock(ts, lock);
+			}
 			thread_lock(td);
 			critical_exit();
 			KASSERT(td->td_locks == locksheld,

Modified: head/sys/kern/subr_turnstile.c
==============================================================================
--- head/sys/kern/subr_turnstile.c	Wed Jul 24 23:04:26 2019	(r350309)
+++ head/sys/kern/subr_turnstile.c	Wed Jul 24 23:04:59 2019	(r350310)
@@ -566,24 +566,26 @@ turnstile_trywait(struct lock_object *lock)
 	return (ts);
 }
 
-struct thread *
-turnstile_lock(struct turnstile *ts, struct lock_object **lockp)
+bool
+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

Modified: head/sys/sys/turnstile.h
==============================================================================
--- head/sys/sys/turnstile.h	Wed Jul 24 23:04:26 2019	(r350309)
+++ head/sys/sys/turnstile.h	Wed Jul 24 23:04:59 2019	(r350310)
@@ -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 */