b_waiters cannot be adequately protected by the interlock because it is

dropped after the call to lockmgr() so just revert this approach using
something similar to the precedent one:
BUF_LOCKWAITERS() just checks if there are waiters (not the actual number
of them) and it is based on newly introduced lockmgr_waiters() which
returns if the lockmgr has waiters or not. The name has been choosen
differently by old lockwaiters() in order to not confuse them.

KPI results enriched by this commit so __FreeBSD_version bumping and
manpage update will be happening soon.
'struct buf' also changes, so kernel ABI is disturbed.

Bug found by:	jeff
Approved by:	jeff, kib

4 files changed, 24 insertions, 52 deletions

diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index cc04f37..a1725be 100644
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -544,7 +544,6 @@ bufinit(void)
 		bp->b_qindex = QUEUE_EMPTY;
 		bp->b_vflags = 0;
 		bp->b_xflags = 0;
-		bp->b_waiters = 0;
 		LIST_INIT(&bp->b_dep);
 		BUF_LOCKINIT(bp);
 		TAILQ_INSERT_TAIL(&bufqueues[QUEUE_EMPTY], bp, b_freelist);
@@ -1183,7 +1182,7 @@ brelse(struct buf *bp)
 			if (bp->b_bufsize)
 				allocbuf(bp, 0);
 			if (bp->b_vp)
-				(void) brelvp(bp);
+				brelvp(bp);
 		}
 	}
 
@@ -1324,7 +1323,7 @@ brelse(struct buf *bp)
 		if (bp->b_bufsize != 0)
 			allocbuf(bp, 0);
 		if (bp->b_vp != NULL)
-			(void) brelvp(bp);
+			brelvp(bp);
 	}
 			
 	if (BUF_LOCKRECURSED(bp)) {
@@ -1388,7 +1387,7 @@ brelse(struct buf *bp)
 		if (bp->b_flags & B_DELWRI)
 			bundirty(bp);
 		if (bp->b_vp)
-			(void) brelvp(bp);
+			brelvp(bp);
 	}
 
 	/*
@@ -1555,7 +1554,7 @@ vfs_vmio_release(struct buf *bp)
 	bp->b_npages = 0;
 	bp->b_flags &= ~B_VMIO;
 	if (bp->b_vp)
-		(void) brelvp(bp);
+		brelvp(bp);
 }
 
 /*
@@ -1693,7 +1692,6 @@ getnewbuf(int slpflag, int slptimeo, int size, int maxsize)
 	struct buf *nbp;
 	int defrag = 0;
 	int nqindex;
-	int waiters = 0;
 	static int flushingbufs;
 
 	/*
@@ -1832,7 +1830,7 @@ restart:
 				vfs_vmio_release(bp);
 			}
 			if (bp->b_vp)
-				waiters = brelvp(bp);
+				brelvp(bp);
 		}
 
 		/*
@@ -1901,7 +1899,7 @@ restart:
 		 * Notify any waiters for the buffer lock about
 		 * identity change by freeing the buffer.
 		 */
-		if (qindex == QUEUE_CLEAN && waiters > 0) {
+		if (qindex == QUEUE_CLEAN && BUF_LOCKWAITERS(bp)) {
 			bp->b_flags |= B_INVAL;
 			bfreekva(bp);
 			brelse(bp);
diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index b3bd6f6..84de803 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -1527,12 +1527,11 @@ bgetvp(struct vnode *vp, struct buf *bp)
 /*
  * Disassociate a buffer from a vnode.
  */
-int
+void
 brelvp(struct buf *bp)
 {
 	struct bufobj *bo;
 	struct vnode *vp;
-	int waiters;
 
 	CTR3(KTR_BUF, "brelvp(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
 	KASSERT(bp->b_vp != NULL, ("brelvp: NULL"));
@@ -1557,11 +1556,8 @@ brelvp(struct buf *bp)
 	bp->b_flags &= ~B_NEEDSGIANT;
 	bp->b_vp = NULL;
 	bp->b_bufobj = NULL;
-	waiters = bp->b_waiters;
 	BO_UNLOCK(bo);
 	vdrop(vp);
-
-	return (waiters);
 }
 
 /*
diff --git a/sys/sys/buf.h b/sys/sys/buf.h
index d735fb7..e05b20c 100644
--- a/sys/sys/buf.h
+++ b/sys/sys/buf.h
@@ -139,7 +139,6 @@ struct buf {
 	void	*b_fsprivate2;
 	void	*b_fsprivate3;
 	int	b_pin_count;
-	int	b_waiters;		/* (V) waiters counter */
 };
 
 #define b_object	b_bufobj->bo_object
@@ -267,38 +266,18 @@ extern const char *buf_wmesg;		/* Default buffer lock message */
  *
  * Get a lock sleeping non-interruptably until it becomes available.
  */
-static __inline int
-_BUF_LOCK(struct buf *bp, int locktype, struct mtx *interlock, char *file,
-    int line)
-{
-	int res;
-
-	if (locktype & LK_INTERLOCK)
-		bp->b_waiters++;
-	res = _lockmgr_args(&bp->b_lock, locktype, interlock, LK_WMESG_DEFAULT,
-	    LK_PRIO_DEFAULT, LK_TIMO_DEFAULT, file, line);
-	if (locktype & LK_INTERLOCK)
-		bp->b_waiters--;
-	return (res);
-}
+#define	BUF_LOCK(bp, locktype, interlock)				\
+	_lockmgr_args(&(bp)->b_lock, (locktype), (interlock),		\
+	    LK_WMESG_DEFAULT, LK_PRIO_DEFAULT, LK_TIMO_DEFAULT,		\
+	    LOCK_FILE, LOCK_LINE)
 
 /*
  * Get a lock sleeping with specified interruptably and timeout.
  */
-static __inline int
-_BUF_TIMELOCK(struct buf *bp, int locktype, struct mtx *interlock,
-    const char *wmesg, int catch, int timo, char *file, int line)
-{
-	int res;
-
-	if (locktype & LK_INTERLOCK)
-		bp->b_waiters++;
-	res = _lockmgr_args(&bp->b_lock, locktype | LK_TIMELOCK, interlock,
-	    wmesg, (PRIBIO + 4) | catch, timo, file, line);
-	if (locktype & LK_INTERLOCK)
-		bp->b_waiters--;
-	return (res);
-}
+#define	BUF_TIMELOCK(bp, locktype, interlock, wmesg, catch, timo)	\
+	_lockmgr_args(&(bp)->b_lock, (locktype) | LK_TIMELOCK,		\
+	    (interlock), (wmesg), (PRIBIO + 4) | (catch), (timo),	\
+	    LOCK_FILE, LOCK_LINE)
 
 /*
  * Release a lock. Only the acquiring process may free the lock unless
@@ -331,15 +310,6 @@ _BUF_TIMELOCK(struct buf *bp, int locktype, struct mtx *interlock,
 	lockdestroy(&(bp)->b_lock)
 
 /*
- * Use macro wrappers in order to exploit consumers tracking.
- */
-#define	BUF_LOCK(bp, locktype, interlock)				\
-	_BUF_LOCK((bp), (locktype), (interlock), LOCK_FILE, LOCK_LINE)
-#define	BUF_TIMELOCK(bp, locktype, interlock, wmesg, catch, timo)	\
-	_BUF_TIMELOCK((bp), (locktype), (interlock), (wmesg), (catch),	\
-	    (timo), LOCK_FILE, LOCK_LINE)
-
-/*
  * Buffer lock assertions.
  */
 #if defined(INVARIANTS) && defined(INVARIANT_SUPPORT)
@@ -373,6 +343,12 @@ _BUF_TIMELOCK(struct buf *bp, int locktype, struct mtx *interlock,
 	_lockmgr_disown(&(bp)->b_lock, LOCK_FILE, LOCK_LINE)
 #endif
 
+/*
+ * Find out if the lock has waiters or not.
+ */
+#define	BUF_LOCKWAITERS(bp)						\
+	lockmgr_waiters(&(bp)->b_lock)
+
 #endif /* _KERNEL */
 
 struct buf_queue_head {
@@ -528,7 +504,7 @@ void	vfs_unbusy_pages(struct buf *);
 int	vmapbuf(struct buf *);
 void	vunmapbuf(struct buf *);
 void	relpbuf(struct buf *, int *);
-int	brelvp(struct buf *);
+void	brelvp(struct buf *);
 void	bgetvp(struct vnode *, struct buf *);
 void	pbgetbo(struct bufobj *bo, struct buf *bp);
 void	pbgetvp(struct vnode *, struct buf *);
diff --git a/sys/sys/lockmgr.h b/sys/sys/lockmgr.h
index 1b89355..9e2fe59 100644
--- a/sys/sys/lockmgr.h
+++ b/sys/sys/lockmgr.h
@@ -225,6 +225,8 @@ int	lockstatus(struct lock *);
 	    LOCK_FILE, LOCK_LINE)
 #define	lockmgr_recursed(lkp)						\
 	((lkp)->lk_exclusivecount > 1)
+#define	lockmgr_waiters(lkp)						\
+	((lkp)->lk_waitcount != 0)
 #ifdef INVARIANTS
 #define	lockmgr_assert(lkp, what)					\
 	_lockmgr_assert((lkp), (what), LOCK_FILE, LOCK_LINE)