[XFS] Plug a pagebuf race that got bigger with the recent cleanup

SGI Modid: xfs-linux:xfs-kern:165722a

fs/xfs/linux/xfs_buf.c

@@ -390,22 +390,24 @@ void
 pagebuf_free(
 	page_buf_t		*pb)
 {
-	page_buf_flags_t	pb_flags = pb->pb_flags;
-	pb_hash_t		*hash;
-
-	PB_TRACE(pb, "free_object", 0);
-	pb->pb_flags |= PBF_FREED;
+	PB_TRACE(pb, "free", 0);
 
 	if (pb->pb_flags & _PBF_LOCKABLE) {
-		hash = pb_hash(pb);
+		 pb_hash_t	*hash = pb_hash(pb);
 
 		spin_lock(&hash->pb_hash_lock);
-		if (!list_empty(&pb->pb_hash_list))
+		/*
+		 * Someone grabbed a reference while we weren't looking,
+		 * try again later.
+		 */
+		if (unlikely(atomic_read(&pb->pb_hold))) {
+			spin_unlock(&hash->pb_hash_lock);
+			return;
+		} else if (!list_empty(&pb->pb_hash_list))
 			list_del_init(&pb->pb_hash_list);
 		spin_unlock(&hash->pb_hash_lock);
 	}
 
-	if (!(pb_flags & PBF_FREED)) {
 	/* release any virtual mapping */ ;
 	if (pb->pb_flags & _PBF_ADDR_ALLOCATED) {
 		void *vaddr = pagebuf_mapout_locked(pb);
@@ -429,7 +431,6 @@ pagebuf_free(
 		}
 		pb->pb_flags &= ~(_PBF_MEM_ALLOCATED|_PBF_MEM_SLAB);
 	}
-	}
 
 	pagebuf_deallocate(pb);
 }
@@ -653,16 +654,15 @@ _pagebuf_find(				/* find buffer for block	*/
 	list_for_each(p, &h->pb_hash) {
 		pb = list_entry(p, page_buf_t, pb_hash_list);
 
-		if ((target == pb->pb_target) &&
-		    (pb->pb_file_offset == range_base) &&
-		    (pb->pb_buffer_length == range_length)) {
-			if (pb->pb_flags & PBF_FREED)
-				break;
+		if (pb->pb_target == target &&
+		    pb->pb_file_offset == range_base &&
+		    pb->pb_buffer_length == range_length &&
+		    atomic_read(&pb->pb_hold)) {
 			/* If we look at something bring it to the
 			 * front of the list for next time
 			 */
-			list_del(&pb->pb_hash_list);
-			list_add(&pb->pb_hash_list, &h->pb_hash);
+			atomic_inc(&pb->pb_hold);
+			list_move(&pb->pb_hash_list, &h->pb_hash);
 			goto found;
 		}
 	}
@@ -681,7 +681,6 @@ _pagebuf_find(				/* find buffer for block	*/
 	return (new_pb);
 
 found:
-	atomic_inc(&pb->pb_hold);
 	spin_unlock(&h->pb_hash_lock);
 
 	/* Attempt to get the semaphore without sleeping,

fs/xfs/linux/xfs_buf.h

@@ -76,7 +76,6 @@ typedef enum page_buf_flags_e {		/* pb_flags values */
 	PBF_ASYNC = (1 << 4),   /* initiator will not wait for completion  */
 	PBF_NONE = (1 << 5),    /* buffer not read at all                  */
 	PBF_DELWRI = (1 << 6),  /* buffer has dirty pages                  */
-	PBF_FREED = (1 << 7),   /* buffer has been freed and is invalid    */
 	PBF_SYNC = (1 << 8),    /* force updates to disk                   */
 	PBF_MAPPABLE = (1 << 9),/* use directly-addressable pages          */
 	PBF_STALE = (1 << 10),	/* buffer has been staled, do not find it  */
@@ -90,7 +89,6 @@ typedef enum page_buf_flags_e {		/* pb_flags values */
 
 	/* flags used only internally */
 	_PBF_LOCKABLE = (1 << 16), /* page_buf_t may be locked		   */
-	_PBF_PRIVATE_BH = (1 << 17), /* do not use public buffer heads	   */
 	_PBF_ALL_PAGES_MAPPED = (1 << 18), /* all pages in range mapped	   */
 	_PBF_ADDR_ALLOCATED = (1 << 19), /* pb_addr space was allocated	   */
 	_PBF_MEM_ALLOCATED = (1 << 20), /* underlying pages are allocated  */