Fix read errors race after block cloning

Investigating read errors triggering panic fixed in openzfs#16042 I've
found that we have a race in a sync process between the moment
dirty record for cloned block is removed and the moment dbuf is
destroyed.  If dmu_buf_hold_array_by_dnode() take a hold on a
cloned dbuf before it is synced/destroyed, then dbuf_read_impl()
may see it still in DB_NOFILL state, but without the dirty record.
Such case is not an error, but equivalent to DB_UNCACHED, since
the dbuf block pointer is already updated by dbuf_write_ready().
Unfortunately it is impossible to safely change the dbuf state
to DB_UNCACHED there, since there may already be another cloning
in progress, that dropped dbuf lock before creating a new dirty
record, protected only by the range lock.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.

module/zfs/dbuf.c

@@ -1564,7 +1564,7 @@ dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags,
 	zbookmark_phys_t zb;
 	uint32_t aflags = ARC_FLAG_NOWAIT;
 	int err, zio_flags;
-	blkptr_t bp, *bpp;
+	blkptr_t bp, *bpp = NULL;
 
 	DB_DNODE_ENTER(db);
 	dn = DB_DNODE(db);
@@ -1580,29 +1580,28 @@ dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags,
 		goto early_unlock;
 	}
 
-	if (db->db_state == DB_UNCACHED) {
-		if (db->db_blkptr == NULL) {
-			bpp = NULL;
-		} else {
-			bp = *db->db_blkptr;
+	/*
+	 * If we have a pending block clone, we don't want to read the
+	 * underlying block, but the content of the block being cloned,
+	 * pointed by the dirty record, so we have the most recent data.
+	 * If there is no dirty record, then we hit a race in a sync
+	 * process when the dirty record is already removed, while the
+	 * dbuf is not yet destroyed. Such case is equivalent to uncached.
+	 */
+	if (db->db_state == DB_NOFILL) {
+		dbuf_dirty_record_t *dr = list_head(&db->db_dirty_records);
+		if (dr != NULL) {
+			if (!dr->dt.dl.dr_brtwrite) {
+				err = EIO;
+				goto early_unlock;
+			}
+			bp = dr->dt.dl.dr_overridden_by;
 			bpp = &bp;
 		}
-	} else {
-		dbuf_dirty_record_t *dr;
-
-		ASSERT3S(db->db_state, ==, DB_NOFILL);
+	}
 
-		/*
-		 * Block cloning: If we have a pending block clone,
-		 * we don't want to read the underlying block, but the content
-		 * of the block being cloned, so we have the most recent data.
-		 */
-		dr = list_head(&db->db_dirty_records);
-		if (dr == NULL || !dr->dt.dl.dr_brtwrite) {
-			err = EIO;
-			goto early_unlock;
-		}
-		bp = dr->dt.dl.dr_overridden_by;
+	if (bpp == NULL && db->db_blkptr != NULL) {
+		bp = *db->db_blkptr;
 		bpp = &bp;
 	}