Revert "Revert "Fix unlink/xattr deadlock""

This reverts commit 53c7411919
effectively reinstating the asynchronous xattr cleanup code.

These Linux changes were reverted because after testing
and careful contemplation I was convinced that due to the
89260a1c8851ce05ea04b23606ba438b271d890 commit they were no
longer required.

Unfortunately, the deadlock described in #1176  was a case
which wasn't considered.  At mount zfs_unlinked_drain() can
occur which will unlink a list of znodes in effectively a
random order which isn't safe.  The only reason it was safe
to originally revert this change was the we could guarantee
that the VFS would always prune the xattr leaves before the
parents.

Therefore, until we can cleanly resolve this deadlock for
all cases we need to keep this change in spite of the xattr
unlink performance penalty associated with it.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1176
Issue #457

module/zfs/zfs_dir.c

@@ -482,57 +482,6 @@ zfs_unlinked_add(znode_t *zp, dmu_tx_t *tx)
 	    zap_add_int(zsb->z_os, zsb->z_unlinkedobj, zp->z_id, tx));
 }
 
-/*
- * Clean up any znodes that had no links when we either crashed or
- * (force) umounted the file system.
- */
-void
-zfs_unlinked_drain(zfs_sb_t *zsb)
-{
-	zap_cursor_t	zc;
-	zap_attribute_t zap;
-	dmu_object_info_t doi;
-	znode_t		*zp;
-	int		error;
-
-	/*
-	 * Interate over the contents of the unlinked set.
-	 */
-	for (zap_cursor_init(&zc, zsb->z_os, zsb->z_unlinkedobj);
-	    zap_cursor_retrieve(&zc, &zap) == 0;
-	    zap_cursor_advance(&zc)) {
-
-		/*
-		 * See what kind of object we have in list
-		 */
-
-		error = dmu_object_info(zsb->z_os, zap.za_first_integer, &doi);
-		if (error != 0)
-			continue;
-
-		ASSERT((doi.doi_type == DMU_OT_PLAIN_FILE_CONTENTS) ||
-		    (doi.doi_type == DMU_OT_DIRECTORY_CONTENTS));
-		/*
-		 * We need to re-mark these list entries for deletion,
-		 * so we pull them back into core and set zp->z_unlinked.
-		 */
-		error = zfs_zget(zsb, zap.za_first_integer, &zp);
-
-		/*
-		 * We may pick up znodes that are already marked for deletion.
-		 * This could happen during the purge of an extended attribute
-		 * directory.  All we need to do is skip over them, since they
-		 * are already in the system marked z_unlinked.
-		 */
-		if (error != 0)
-			continue;
-
-		zp->z_unlinked = B_TRUE;
-		iput(ZTOI(zp));
-	}
-	zap_cursor_fini(&zc);
-}
-
 /*
  * Delete the entire contents of a directory.  Return a count
  * of the number of entries that could not be deleted. If we encounter
@@ -599,6 +548,71 @@ zfs_purgedir(znode_t *dzp)
 	return (skipped);
 }
 
+/*
+ * Clean up any znodes that had no links when we either crashed or
+ * (force) umounted the file system.
+ */
+void
+zfs_unlinked_drain(zfs_sb_t *zsb)
+{
+	zap_cursor_t	zc;
+	zap_attribute_t zap;
+	dmu_object_info_t doi;
+	znode_t		*zp;
+	int		error;
+
+	/*
+	 * Interate over the contents of the unlinked set.
+	 */
+	for (zap_cursor_init(&zc, zsb->z_os, zsb->z_unlinkedobj);
+	    zap_cursor_retrieve(&zc, &zap) == 0;
+	    zap_cursor_advance(&zc)) {
+
+		/*
+		 * See what kind of object we have in list
+		 */
+
+		error = dmu_object_info(zsb->z_os, zap.za_first_integer, &doi);
+		if (error != 0)
+			continue;
+
+		ASSERT((doi.doi_type == DMU_OT_PLAIN_FILE_CONTENTS) ||
+		    (doi.doi_type == DMU_OT_DIRECTORY_CONTENTS));
+		/*
+		 * We need to re-mark these list entries for deletion,
+		 * so we pull them back into core and set zp->z_unlinked.
+		 */
+		error = zfs_zget(zsb, zap.za_first_integer, &zp);
+
+		/*
+		 * We may pick up znodes that are already marked for deletion.
+		 * This could happen during the purge of an extended attribute
+		 * directory.  All we need to do is skip over them, since they
+		 * are already in the system marked z_unlinked.
+		 */
+		if (error != 0)
+			continue;
+
+		zp->z_unlinked = B_TRUE;
+
+		/*
+		 * If this is an attribute directory, purge its contents.
+		 */
+		if (S_ISDIR(ZTOI(zp)->i_mode) && (zp->z_pflags & ZFS_XATTR)) {
+			/*
+			 * We don't need to check the return value of
+			 * zfs_purgedir here, because zfs_rmnode will just
+			 * return this xattr directory to the unlinked set
+			 * until all of its xattrs are gone.
+			 */
+			(void) zfs_purgedir(zp);
+		}
+
+		iput(ZTOI(zp));
+	}
+	zap_cursor_fini(&zc);
+}
+
 void
 zfs_rmnode(znode_t *zp)
 {
@@ -608,6 +622,7 @@ zfs_rmnode(znode_t *zp)
 	dmu_tx_t	*tx;
 	uint64_t	acl_obj;
 	uint64_t	xattr_obj;
+	uint64_t	count;
 	int		error;
 
 	ASSERT(zp->z_links == 0);
@@ -617,13 +632,27 @@ zfs_rmnode(znode_t *zp)
 	 * If this is an attribute directory, purge its contents.
 	 */
 	if (S_ISDIR(ZTOI(zp)->i_mode) && (zp->z_pflags & ZFS_XATTR)) {
-		if (zfs_purgedir(zp) != 0) {
+		error = zap_count(os, zp->z_id, &count);
-			/*
+		if (error) {
-			 * Not enough space to delete some xattrs.
-			 * Leave it in the unlinked set.
-			 */
 			zfs_znode_dmu_fini(zp);
+			return;
+		}
 
+		if (count > 0) {
+			taskq_t *taskq;
+
+			/*
+			 * There are still directory entries in this xattr
+			 * directory.  Let zfs_unlinked_drain() deal with
+			 * them to avoid deadlocking this process in the
+			 * zfs_purgedir()->zfs_zget()->ilookup() callpath
+			 * on the xattr inode's I_FREEING bit.
+			 */
+			taskq = dsl_pool_iput_taskq(dmu_objset_pool(os));
+			taskq_dispatch(taskq, (task_func_t *)
+			    zfs_unlinked_drain, zsb, TQ_SLEEP);
+
+			zfs_znode_dmu_fini(zp);
 			return;
 		}
 	}

module/zfs/zfs_vfsops.c

@@ -1055,6 +1055,12 @@ zfs_sb_teardown(zfs_sb_t *zsb, boolean_t unmounting)
 		shrink_dcache_sb(zsb->z_parent->z_sb);
 	}
 
+	/*
+	 * Drain the iput_taskq to ensure all active references to the
+	 * zfs_sb_t have been handled only then can it be safely destroyed.
+	 */
+	taskq_wait(dsl_pool_iput_taskq(dmu_objset_pool(zsb->z_os)));
+
 	/*
 	 * Close the zil. NB: Can't close the zil while zfs_inactive
 	 * threads are blocked as zil_close can call zfs_inactive.