From: Matthew Ahrens <mahrens@delphix.com>
Date: Mon, 29 Aug 2016 18:40:16 +0000 (-0700)
Subject: OpenZFS 7086 - ztest attempts dva_get_dsize_sync on an embedded blockpointer
X-Git-Tag: zfs-0.7.0-rc1~16
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=98ace739bd89b541af30d9d627ee42622fbbd861;p=zfs

OpenZFS 7086 - ztest attempts dva_get_dsize_sync on an embedded blockpointer

In dbuf_dirty(), we need to grab the dn_struct_rwlock before looking at
the db_blkptr, to prevent it from being changed by syncing context.

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7086
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/98fa317
Closes #5039
---

diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
index d297506e9..e452fd23a 100644
--- a/module/zfs/dbuf.c
+++ b/module/zfs/dbuf.c
@@ -1468,7 +1468,20 @@ dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
 		dnode_setdirty(dn, tx);
 		DB_DNODE_EXIT(db);
 		return (dr);
-	} else if (do_free_accounting) {
+	}
+
+	/*
+	 * The dn_struct_rwlock prevents db_blkptr from changing
+	 * due to a write from syncing context completing
+	 * while we are running, so we want to acquire it before
+	 * looking at db_blkptr.
+	 */
+	if (!RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
+		rw_enter(&dn->dn_struct_rwlock, RW_READER);
+		drop_struct_lock = TRUE;
+	}
+
+	if (do_free_accounting) {
 		blkptr_t *bp = db->db_blkptr;
 		int64_t willfree = (bp && !BP_IS_HOLE(bp)) ?
 		    bp_get_dsize(os->os_spa, bp) : db->db.db_size;
@@ -1484,11 +1497,6 @@ dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
 		dnode_willuse_space(dn, -willfree, tx);
 	}
 
-	if (!RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
-		rw_enter(&dn->dn_struct_rwlock, RW_READER);
-		drop_struct_lock = TRUE;
-	}
-
 	if (db->db_level == 0) {
 		dnode_new_blkid(dn, db->db_blkid, tx, drop_struct_lock);
 		ASSERT(dn->dn_maxblkid >= db->db_blkid);