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changeset 23893:49e0cb1642f6

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13317 Decrease contention on dn_struct_rwlock Portions contributed by: Jason King <jason.king@joyent.com> Reviewed by: Brad Lewis <brad.lewis@delphix.com> Reviewed by: Matt Ahrens matt@delphix.com Reviewed by: George Wilson george.wilson@delphix.com Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov> Approved by: Dan McDonald <danmcd@joyent.com>
author Paul Dagnelie <pcd@delphix.com>
date Tue, 17 Nov 2020 10:47:03 -0600
parents 790618c19823
children 9fc483dacaa9
files usr/src/uts/common/fs/zfs/dbuf.c usr/src/uts/common/fs/zfs/dmu.c usr/src/uts/common/fs/zfs/dmu_zfetch.c usr/src/uts/common/fs/zfs/dnode.c usr/src/uts/common/fs/zfs/dnode_sync.c usr/src/uts/common/fs/zfs/sys/dbuf.h usr/src/uts/common/fs/zfs/sys/dmu_zfetch.h
diffstat 7 files changed, 247 insertions(+), 121 deletions(-) [+]
line wrap: on
line diff
--- a/usr/src/uts/common/fs/zfs/dbuf.c	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/dbuf.c	Tue Nov 17 10:47:03 2020 -0600
@@ -21,7 +21,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
- * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2019 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
  * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
@@ -176,6 +176,7 @@
 	bzero(db, sizeof (dmu_buf_impl_t));
 
 	mutex_init(&db->db_mtx, NULL, MUTEX_DEFAULT, NULL);
+	rw_init(&db->db_rwlock, NULL, RW_DEFAULT, NULL);
 	cv_init(&db->db_changed, NULL, CV_DEFAULT, NULL);
 	multilist_link_init(&db->db_cache_link);
 	zfs_refcount_create(&db->db_holds);
@@ -189,6 +190,7 @@
 {
 	dmu_buf_impl_t *db = vdb;
 	mutex_destroy(&db->db_mtx);
+	rw_destroy(&db->db_rwlock);
 	cv_destroy(&db->db_changed);
 	ASSERT(!multilist_link_active(&db->db_cache_link));
 	zfs_refcount_destroy(&db->db_holds);
@@ -789,10 +791,10 @@
 			    db->db.db_object);
 			/*
 			 * dnode_grow_indblksz() can make this fail if we don't
-			 * have the struct_rwlock.  XXX indblksz no longer
+			 * have the parent's rwlock.  XXX indblksz no longer
 			 * grows.  safe to do this now?
 			 */
-			if (RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
+			if (RW_LOCK_HELD(&db->db_parent->db_rwlock)) {
 				ASSERT3P(db->db_blkptr, ==,
 				    ((blkptr_t *)db->db_parent->db.db_data +
 				    db->db_blkid % epb));
@@ -868,6 +870,44 @@
 		db->db_state = DB_UNCACHED;
 }
 
+/*
+ * This function is used to lock the parent of the provided dbuf. This should be
+ * used when modifying or reading db_blkptr.
+ */
+db_lock_type_t
+dmu_buf_lock_parent(dmu_buf_impl_t *db, krw_t rw, void *tag)
+{
+	enum db_lock_type ret = DLT_NONE;
+	if (db->db_parent != NULL) {
+		rw_enter(&db->db_parent->db_rwlock, rw);
+		ret = DLT_PARENT;
+	} else if (dmu_objset_ds(db->db_objset) != NULL) {
+		rrw_enter(&dmu_objset_ds(db->db_objset)->ds_bp_rwlock, rw,
+		    tag);
+		ret = DLT_OBJSET;
+	}
+	/*
+	 * We only return a DLT_NONE lock when it's the top-most indirect block
+	 * of the meta-dnode of the MOS.
+	 */
+	return (ret);
+}
+
+/*
+ * We need to pass the lock type in because it's possible that the block will
+ * move from being the topmost indirect block in a dnode (and thus, have no
+ * parent) to not the top-most via an indirection increase. This would cause a
+ * panic if we didn't pass the lock type in.
+ */
+void
+dmu_buf_unlock_parent(dmu_buf_impl_t *db, db_lock_type_t type, void *tag)
+{
+	if (type == DLT_PARENT)
+		rw_exit(&db->db_parent->db_rwlock);
+	else if (type == DLT_OBJSET)
+		rrw_exit(&dmu_objset_ds(db->db_objset)->ds_bp_rwlock, tag);
+}
+
 static void
 dbuf_set_data(dmu_buf_impl_t *db, arc_buf_t *buf)
 {
@@ -1042,8 +1082,13 @@
 	return (err);
 }
 
+/*
+ * Drops db_mtx and the parent lock specified by dblt and tag before
+ * returning.
+ */
 static int
-dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
+dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags,
+    db_lock_type_t dblt, void *tag)
 {
 	dnode_t *dn;
 	zbookmark_phys_t zb;
@@ -1053,11 +1098,11 @@
 	DB_DNODE_ENTER(db);
 	dn = DB_DNODE(db);
 	ASSERT(!zfs_refcount_is_zero(&db->db_holds));
-	/* We need the struct_rwlock to prevent db_blkptr from changing. */
-	ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
 	ASSERT(MUTEX_HELD(&db->db_mtx));
 	ASSERT(db->db_state == DB_UNCACHED);
 	ASSERT(db->db_buf == NULL);
+	ASSERT(db->db_parent == NULL ||
+	    RW_LOCK_HELD(&db->db_parent->db_rwlock));
 
 	if (db->db_blkid == DMU_BONUS_BLKID) {
 		/*
@@ -1085,6 +1130,7 @@
 		DB_DNODE_EXIT(db);
 		db->db_state = DB_CACHED;
 		mutex_exit(&db->db_mtx);
+		dmu_buf_unlock_parent(db, dblt, tag);
 		return (0);
 	}
 
@@ -1125,6 +1171,7 @@
 		DB_DNODE_EXIT(db);
 		db->db_state = DB_CACHED;
 		mutex_exit(&db->db_mtx);
+		dmu_buf_unlock_parent(db, dblt, tag);
 		return (0);
 	}
 
@@ -1141,12 +1188,14 @@
 		    "object set %llu", dmu_objset_id(db->db_objset));
 		DB_DNODE_EXIT(db);
 		mutex_exit(&db->db_mtx);
+		dmu_buf_unlock_parent(db, dblt, tag);
 		return (SET_ERROR(EIO));
 	}
 
 	err = dbuf_read_verify_dnode_crypt(db, flags);
 	if (err != 0) {
 		DB_DNODE_EXIT(db);
+		dmu_buf_unlock_parent(db, dblt, tag);
 		mutex_exit(&db->db_mtx);
 		return (err);
 	}
@@ -1166,11 +1215,18 @@
 
 	if ((flags & DB_RF_NO_DECRYPT) && BP_IS_PROTECTED(db->db_blkptr))
 		zio_flags |= ZIO_FLAG_RAW;
-
-	err = arc_read(zio, db->db_objset->os_spa, db->db_blkptr,
+	/*
+	 * The zio layer will copy the provided blkptr later, but we need to
+	 * do this now so that we can release the parent's rwlock. We have to
+	 * do that now so that if dbuf_read_done is called synchronously (on
+	 * an l1 cache hit) we don't acquire the db_mtx while holding the
+	 * parent's rwlock, which would be a lock ordering violation.
+	 */
+	blkptr_t bp = *db->db_blkptr;
+	dmu_buf_unlock_parent(db, dblt, tag);
+	(void) arc_read(zio, db->db_objset->os_spa, &bp,
 	    dbuf_read_done, db, ZIO_PRIORITY_SYNC_READ, zio_flags,
 	    &aflags, &zb);
-
 	return (err);
 }
 
@@ -1269,8 +1325,6 @@
 
 	DB_DNODE_ENTER(db);
 	dn = DB_DNODE(db);
-	if ((flags & DB_RF_HAVESTRUCT) == 0)
-		rw_enter(&dn->dn_struct_rwlock, RW_READER);
 
 	prefetch = db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID &&
 	    (flags & DB_RF_NOPREFETCH) == 0 && dn != NULL &&
@@ -1307,29 +1361,32 @@
 			dbuf_set_data(db, db->db_buf);
 		}
 		mutex_exit(&db->db_mtx);
-		if (err == 0 && prefetch)
-			dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE);
-		if ((flags & DB_RF_HAVESTRUCT) == 0)
-			rw_exit(&dn->dn_struct_rwlock);
+		if (err == 0 && prefetch) {
+			dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE,
+			    flags & DB_RF_HAVESTRUCT);
+		}
 		DB_DNODE_EXIT(db);
 	} else if (db->db_state == DB_UNCACHED) {
 		spa_t *spa = dn->dn_objset->os_spa;
 		boolean_t need_wait = B_FALSE;
 
+		db_lock_type_t dblt = dmu_buf_lock_parent(db, RW_READER, FTAG);
+
 		if (zio == NULL &&
 		    db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)) {
 			zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
 			need_wait = B_TRUE;
 		}
-		err = dbuf_read_impl(db, zio, flags);
-
-		/* dbuf_read_impl has dropped db_mtx for us */
-
-		if (!err && prefetch)
-			dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE);
-
-		if ((flags & DB_RF_HAVESTRUCT) == 0)
-			rw_exit(&dn->dn_struct_rwlock);
+		err = dbuf_read_impl(db, zio, flags, dblt, FTAG);
+		/*
+		 * dbuf_read_impl has dropped db_mtx and our parent's rwlock
+		 * for us
+		 */
+		if (!err && prefetch) {
+			dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE,
+			    flags & DB_RF_HAVESTRUCT);
+		}
+
 		DB_DNODE_EXIT(db);
 
 		if (!err && need_wait)
@@ -1344,10 +1401,10 @@
 		 * occurred and the dbuf went to UNCACHED.
 		 */
 		mutex_exit(&db->db_mtx);
-		if (prefetch)
-			dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE);
-		if ((flags & DB_RF_HAVESTRUCT) == 0)
-			rw_exit(&dn->dn_struct_rwlock);
+		if (prefetch) {
+			dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE,
+			    flags & DB_RF_HAVESTRUCT);
+		}
 		DB_DNODE_EXIT(db);
 
 		/* Skip the wait per the caller's request. */
@@ -1527,7 +1584,9 @@
 		if (db->db_state == DB_CACHED) {
 			ASSERT(db->db.db_data != NULL);
 			arc_release(db->db_buf, db);
+			rw_enter(&db->db_rwlock, RW_WRITER);
 			bzero(db->db.db_data, db->db.db_size);
+			rw_exit(&db->db_rwlock);
 			arc_buf_freeze(db->db_buf);
 		}
 
@@ -1549,15 +1608,6 @@
 	DB_DNODE_ENTER(db);
 	dn = DB_DNODE(db);
 
-	/* XXX does *this* func really need the lock? */
-	ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));
-
-	/*
-	 * This call to dmu_buf_will_dirty() with the dn_struct_rwlock held
-	 * is OK, because there can be no other references to the db
-	 * when we are changing its size, so no concurrent DB_FILL can
-	 * be happening.
-	 */
 	/*
 	 * XXX we should be doing a dbuf_read, checking the return
 	 * value and returning that up to our callers
@@ -1634,8 +1684,8 @@
 	dnode_t *dn;
 	objset_t *os;
 	dbuf_dirty_record_t **drp, *dr;
-	int drop_struct_lock = FALSE;
 	int txgoff = tx->tx_txg & TXG_MASK;
+	boolean_t drop_struct_rwlock = B_FALSE;
 
 	ASSERT(tx->tx_txg != 0);
 	ASSERT(!zfs_refcount_is_zero(&db->db_holds));
@@ -1837,15 +1887,21 @@
 		return (dr);
 	}
 
-	/*
-	 * 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;
+		drop_struct_rwlock = B_TRUE;
+	}
+
+	/*
+	 * If we are overwriting a dedup BP, then unless it is snapshotted,
+	 * when we get to syncing context we will need to decrement its
+	 * refcount in the DDT.  Prefetch the relevant DDT block so that
+	 * syncing context won't have to wait for the i/o.
+	 */
+	if (db->db_blkptr != NULL) {
+		db_lock_type_t dblt = dmu_buf_lock_parent(db, RW_READER, FTAG);
+		ddt_prefetch(os->os_spa, db->db_blkptr);
+		dmu_buf_unlock_parent(db, dblt, FTAG);
 	}
 
 	/*
@@ -1858,19 +1914,12 @@
 	    dn->dn_next_nlevels[(tx->tx_txg-1) & TXG_MASK] > db->db_level ||
 	    dn->dn_next_nlevels[(tx->tx_txg-2) & TXG_MASK] > db->db_level);
 
-	/*
-	 * If we are overwriting a dedup BP, then unless it is snapshotted,
-	 * when we get to syncing context we will need to decrement its
-	 * refcount in the DDT.  Prefetch the relevant DDT block so that
-	 * syncing context won't have to wait for the i/o.
-	 */
-	ddt_prefetch(os->os_spa, db->db_blkptr);
 
 	if (db->db_level == 0) {
 		ASSERT(!db->db_objset->os_raw_receive ||
 		    dn->dn_maxblkid >= db->db_blkid);
 		dnode_new_blkid(dn, db->db_blkid, tx,
-		    drop_struct_lock, B_FALSE);
+		    drop_struct_rwlock, B_FALSE);
 		ASSERT(dn->dn_maxblkid >= db->db_blkid);
 	}
 
@@ -1881,15 +1930,14 @@
 
 		if (db->db_parent == NULL || db->db_parent == dn->dn_dbuf) {
 			int epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
-
-			parent = dbuf_hold_level(dn, db->db_level+1,
+			parent = dbuf_hold_level(dn, db->db_level + 1,
 			    db->db_blkid >> epbs, FTAG);
 			ASSERT(parent != NULL);
 			parent_held = TRUE;
 		}
-		if (drop_struct_lock)
+		if (drop_struct_rwlock)
 			rw_exit(&dn->dn_struct_rwlock);
-		ASSERT3U(db->db_level+1, ==, parent->db_level);
+		ASSERT3U(db->db_level + 1, ==, parent->db_level);
 		di = dbuf_dirty(parent, tx);
 		if (parent_held)
 			dbuf_rele(parent, FTAG);
@@ -1910,14 +1958,14 @@
 		}
 		mutex_exit(&db->db_mtx);
 	} else {
-		ASSERT(db->db_level+1 == dn->dn_nlevels);
+		ASSERT(db->db_level + 1 == dn->dn_nlevels);
 		ASSERT(db->db_blkid < dn->dn_nblkptr);
 		ASSERT(db->db_parent == NULL || db->db_parent == dn->dn_dbuf);
 		mutex_enter(&dn->dn_mtx);
 		ASSERT(!list_link_active(&dr->dr_dirty_node));
 		list_insert_tail(&dn->dn_dirty_records[txgoff], dr);
 		mutex_exit(&dn->dn_mtx);
-		if (drop_struct_lock)
+		if (drop_struct_rwlock)
 			rw_exit(&dn->dn_struct_rwlock);
 	}
 
@@ -2438,10 +2486,12 @@
 			*parentp = NULL;
 			return (err);
 		}
+		rw_enter(&(*parentp)->db_rwlock, RW_READER);
 		*bpp = ((blkptr_t *)(*parentp)->db.db_data) +
 		    (blkid & ((1ULL << epbs) - 1));
 		if (blkid > (dn->dn_phys->dn_maxblkid >> (level * epbs)))
 			ASSERT(BP_IS_HOLE(*bpp));
+		rw_exit(&(*parentp)->db_rwlock);
 		return (0);
 	} else {
 		/* the block is referenced from the dnode */
@@ -2686,7 +2736,7 @@
 	if (blkid > dn->dn_maxblkid)
 		return;
 
-	if (dnode_block_freed(dn, blkid))
+	if (level == 0 && dnode_block_freed(dn, blkid))
 		return;
 
 	/*
@@ -2832,7 +2882,9 @@
 		    DBUF_GET_BUFC_TYPE(db), db->db.db_size));
 	}
 
+	rw_enter(&db->db_rwlock, RW_WRITER);
 	bcopy(data->b_data, db->db.db_data, arc_buf_size(data));
+	rw_exit(&db->db_rwlock);
 }
 
 /*
@@ -2958,7 +3010,6 @@
 dbuf_spill_set_blksz(dmu_buf_t *db_fake, uint64_t blksz, dmu_tx_t *tx)
 {
 	dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
-	dnode_t *dn;
 
 	if (db->db_blkid != DMU_SPILL_BLKID)
 		return (SET_ERROR(ENOTSUP));
@@ -2967,12 +3018,7 @@
 	ASSERT3U(blksz, <=, spa_maxblocksize(dmu_objset_spa(db->db_objset)));
 	blksz = P2ROUNDUP(blksz, SPA_MINBLOCKSIZE);
 
-	DB_DNODE_ENTER(db);
-	dn = DB_DNODE(db);
-	rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
 	dbuf_new_size(db, blksz, tx);
-	rw_exit(&dn->dn_struct_rwlock);
-	DB_DNODE_EXIT(db);
 
 	return (0);
 }
@@ -3688,9 +3734,9 @@
 
 	mutex_exit(&db->db_mtx);
 
-	rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
+	db_lock_type_t dblt = dmu_buf_lock_parent(db, RW_WRITER, FTAG);
 	*db->db_blkptr = *bp;
-	rw_exit(&dn->dn_struct_rwlock);
+	dmu_buf_unlock_parent(db, dblt, FTAG);
 }
 
 /* ARGSUSED */
@@ -3731,9 +3777,9 @@
 		 * anybody from reading the blocks we're about to
 		 * zero out.
 		 */
-		rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
+		rw_enter(&db->db_rwlock, RW_WRITER);
 		bzero(db->db.db_data, db->db.db_size);
-		rw_exit(&dn->dn_struct_rwlock);
+		rw_exit(&db->db_rwlock);
 	}
 	DB_DNODE_EXIT(db);
 }
@@ -3923,7 +3969,7 @@
 }
 
 static void
-dbuf_remap_impl(dnode_t *dn, blkptr_t *bp, dmu_tx_t *tx)
+dbuf_remap_impl(dnode_t *dn, blkptr_t *bp, krwlock_t *rw, dmu_tx_t *tx)
 {
 	blkptr_t bp_copy = *bp;
 	spa_t *spa = dmu_objset_spa(dn->dn_objset);
@@ -3937,14 +3983,16 @@
 	if (spa_remap_blkptr(spa, &bp_copy, dbuf_remap_impl_callback,
 	    &drica)) {
 		/*
-		 * The struct_rwlock prevents dbuf_read_impl() from
+		 * The db_rwlock prevents dbuf_read_impl() from
 		 * dereferencing the BP while we are changing it.  To
 		 * avoid lock contention, only grab it when we are actually
 		 * changing the BP.
 		 */
-		rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
+		if (rw != NULL)
+			rw_enter(rw, RW_WRITER);
 		*bp = bp_copy;
-		rw_exit(&dn->dn_struct_rwlock);
+		if (rw != NULL)
+			rw_exit(rw);
 	}
 }
 
@@ -4017,7 +4065,7 @@
 	if (db->db_level > 0) {
 		blkptr_t *bp = db->db.db_data;
 		for (int i = 0; i < db->db.db_size >> SPA_BLKPTRSHIFT; i++) {
-			dbuf_remap_impl(dn, &bp[i], tx);
+			dbuf_remap_impl(dn, &bp[i], &db->db_rwlock, tx);
 		}
 	} else if (db->db.db_object == DMU_META_DNODE_OBJECT) {
 		dnode_phys_t *dnp = db->db.db_data;
@@ -4025,7 +4073,10 @@
 		    DMU_OT_DNODE);
 		for (int i = 0; i < db->db.db_size >> DNODE_SHIFT; i++) {
 			for (int j = 0; j < dnp[i].dn_nblkptr; j++) {
-				dbuf_remap_impl(dn, &dnp[i].dn_blkptr[j], tx);
+				krwlock_t *lock = (dn->dn_dbuf == NULL ? NULL :
+				    &dn->dn_dbuf->db_rwlock);
+				dbuf_remap_impl(dn, &dnp[i].dn_blkptr[j], lock,
+				    tx);
 			}
 		}
 	}
--- a/usr/src/uts/common/fs/zfs/dmu.c	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/dmu.c	Tue Nov 17 10:47:03 2020 -0600
@@ -172,8 +172,8 @@
 	uint64_t blkid;
 	dmu_buf_impl_t *db;
 
+	rw_enter(&dn->dn_struct_rwlock, RW_READER);
 	blkid = dbuf_whichblock(dn, 0, offset);
-	rw_enter(&dn->dn_struct_rwlock, RW_READER);
 	db = dbuf_hold(dn, blkid, tag);
 	rw_exit(&dn->dn_struct_rwlock);
 
@@ -197,8 +197,8 @@
 	err = dnode_hold(os, object, FTAG, &dn);
 	if (err)
 		return (err);
+	rw_enter(&dn->dn_struct_rwlock, RW_READER);
 	blkid = dbuf_whichblock(dn, 0, offset);
-	rw_enter(&dn->dn_struct_rwlock, RW_READER);
 	db = dbuf_hold(dn, blkid, tag);
 	rw_exit(&dn->dn_struct_rwlock);
 	dnode_rele(dn, FTAG);
@@ -605,7 +605,7 @@
 	if ((flags & DMU_READ_NO_PREFETCH) == 0 &&
 	    DNODE_META_IS_CACHEABLE(dn) && length <= zfetch_array_rd_sz) {
 		dmu_zfetch(&dn->dn_zfetch, blkid, nblks,
-		    read && DNODE_IS_CACHEABLE(dn));
+		    read && DNODE_IS_CACHEABLE(dn), B_TRUE);
 	}
 	rw_exit(&dn->dn_struct_rwlock);
 
@@ -737,7 +737,6 @@
 	if (err != 0)
 		return;
 
-	rw_enter(&dn->dn_struct_rwlock, RW_READER);
 	/*
 	 * offset + len - 1 is the last byte we want to prefetch for, and offset
 	 * is the first.  Then dbuf_whichblk(dn, level, off + len - 1) is the
@@ -745,6 +744,7 @@
 	 * offset)  is the first.  Then the number we need to prefetch is the
 	 * last - first + 1.
 	 */
+	rw_enter(&dn->dn_struct_rwlock, RW_READER);
 	if (level > 0 || dn->dn_datablkshift != 0) {
 		nblks = dbuf_whichblock(dn, level, offset + len - 1) -
 		    dbuf_whichblock(dn, level, offset) + 1;
@@ -757,7 +757,6 @@
 		for (int i = 0; i < nblks; i++)
 			dbuf_prefetch(dn, level, blkid + i, pri, 0);
 	}
-
 	rw_exit(&dn->dn_struct_rwlock);
 
 	dnode_rele(dn, FTAG);
--- a/usr/src/uts/common/fs/zfs/dmu_zfetch.c	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/dmu_zfetch.c	Tue Nov 17 10:47:03 2020 -0600
@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2013, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2013, 2017 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
@@ -204,7 +204,8 @@
  *   TRUE -- prefetch predicted data blocks plus following indirect blocks.
  */
 void
-dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data)
+dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data,
+    boolean_t have_lock)
 {
 	zstream_t *zs;
 	int64_t pf_start, ipf_start, ipf_istart, ipf_iend;
@@ -233,6 +234,9 @@
 	if (blkid == 0)
 		return;
 
+	if (!have_lock)
+		rw_enter(&zf->zf_dnode->dn_struct_rwlock, RW_READER);
+
 	rw_enter(&zf->zf_rwlock, RW_READER);
 
 	/*
@@ -257,6 +261,10 @@
 					/* Already prefetched this before. */
 					mutex_exit(&zs->zs_lock);
 					rw_exit(&zf->zf_rwlock);
+					if (!have_lock) {
+						rw_exit(&zf->zf_dnode->
+						    dn_struct_rwlock);
+					}
 					return;
 				}
 				break;
@@ -274,6 +282,8 @@
 		if (rw_tryupgrade(&zf->zf_rwlock))
 			dmu_zfetch_stream_create(zf, end_of_access_blkid);
 		rw_exit(&zf->zf_rwlock);
+		if (!have_lock)
+			rw_exit(&zf->zf_dnode->dn_struct_rwlock);
 		return;
 	}
 
@@ -353,5 +363,7 @@
 		dbuf_prefetch(zf->zf_dnode, 1, iblk,
 		    ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
 	}
+	if (!have_lock)
+		rw_exit(&zf->zf_dnode->dn_struct_rwlock);
 	ZFETCHSTAT_BUMP(zfetchstat_hits);
 }
--- a/usr/src/uts/common/fs/zfs/dnode.c	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/dnode.c	Tue Nov 17 10:47:03 2020 -0600
@@ -1346,7 +1346,6 @@
 	}
 
 	blk = dbuf_whichblock(mdn, 0, object * sizeof (dnode_phys_t));
-
 	db = dbuf_hold(mdn, blk, FTAG);
 	if (drop_struct_lock)
 		rw_exit(&mdn->dn_struct_rwlock);
@@ -1783,10 +1782,11 @@
 
 	/* resize the old block */
 	err = dbuf_hold_impl(dn, 0, 0, TRUE, FALSE, FTAG, &db);
-	if (err == 0)
+	if (err == 0) {
 		dbuf_new_size(db, size, tx);
-	else if (err != ENOENT)
+	} else if (err != ENOENT) {
 		goto fail;
+	}
 
 	dnode_setdblksz(dn, size);
 	dnode_setdirty(dn, tx);
@@ -2021,7 +2021,6 @@
 	int trunc = FALSE;
 	int epbs;
 
-	rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
 	blksz = dn->dn_datablksz;
 	blkshift = dn->dn_datablkshift;
 	epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;
@@ -2038,7 +2037,7 @@
 		head = P2NPHASE(off, blksz);
 		blkoff = P2PHASE(off, blksz);
 		if ((off >> blkshift) > dn->dn_maxblkid)
-			goto out;
+			return;
 	} else {
 		ASSERT(dn->dn_maxblkid == 0);
 		if (off == 0 && len >= blksz) {
@@ -2047,12 +2046,15 @@
 			 */
 			blkid = 0;
 			nblks = 1;
-			if (dn->dn_nlevels > 1)
+			if (dn->dn_nlevels > 1) {
+				rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
 				dnode_dirty_l1(dn, 0, tx);
+				rw_exit(&dn->dn_struct_rwlock);
+			}
 			goto done;
 		} else if (off >= blksz) {
 			/* Freeing past end-of-data */
-			goto out;
+			return;
 		} else {
 			/* Freeing part of the block. */
 			head = blksz - off;
@@ -2062,19 +2064,26 @@
 	}
 	/* zero out any partial block data at the start of the range */
 	if (head) {
+		int res;
 		ASSERT3U(blkoff + head, ==, blksz);
 		if (len < head)
 			head = len;
-		if (dbuf_hold_impl(dn, 0, dbuf_whichblock(dn, 0, off),
-		    TRUE, FALSE, FTAG, &db) == 0) {
+		rw_enter(&dn->dn_struct_rwlock, RW_READER);
+		res = dbuf_hold_impl(dn, 0, dbuf_whichblock(dn, 0, off),
+		    TRUE, FALSE, FTAG, &db);
+		rw_exit(&dn->dn_struct_rwlock);
+		if (res == 0) {
 			caddr_t data;
+			boolean_t dirty;
 
+			db_lock_type_t dblt = dmu_buf_lock_parent(db, RW_READER,
+			    FTAG);
 			/* don't dirty if it isn't on disk and isn't dirty */
-			if (db->db_last_dirty ||
-			    (db->db_blkptr && !BP_IS_HOLE(db->db_blkptr))) {
-				rw_exit(&dn->dn_struct_rwlock);
+			dirty = db->db_last_dirty ||
+			    (db->db_blkptr && !BP_IS_HOLE(db->db_blkptr));
+			dmu_buf_unlock_parent(db, dblt, FTAG);
+			if (dirty) {
 				dmu_buf_will_dirty(&db->db, tx);
-				rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
 				data = db->db.db_data;
 				bzero(data + blkoff, head);
 			}
@@ -2086,11 +2095,11 @@
 
 	/* If the range was less than one block, we're done */
 	if (len == 0)
-		goto out;
+		return;
 
 	/* If the remaining range is past end of file, we're done */
 	if ((off >> blkshift) > dn->dn_maxblkid)
-		goto out;
+		return;
 
 	ASSERT(ISP2(blksz));
 	if (trunc)
@@ -2101,16 +2110,23 @@
 	ASSERT0(P2PHASE(off, blksz));
 	/* zero out any partial block data at the end of the range */
 	if (tail) {
+		int res;
 		if (len < tail)
 			tail = len;
-		if (dbuf_hold_impl(dn, 0, dbuf_whichblock(dn, 0, off+len),
-		    TRUE, FALSE, FTAG, &db) == 0) {
+		rw_enter(&dn->dn_struct_rwlock, RW_READER);
+		res = dbuf_hold_impl(dn, 0, dbuf_whichblock(dn, 0, off+len),
+		    TRUE, FALSE, FTAG, &db);
+		rw_exit(&dn->dn_struct_rwlock);
+		if (res == 0) {
+			boolean_t dirty;
 			/* don't dirty if not on disk and not dirty */
-			if (db->db_last_dirty ||
-			    (db->db_blkptr && !BP_IS_HOLE(db->db_blkptr))) {
-				rw_exit(&dn->dn_struct_rwlock);
+			db_lock_type_t type = dmu_buf_lock_parent(db, RW_READER,
+			    FTAG);
+			dirty = db->db_last_dirty ||
+			    (db->db_blkptr && !BP_IS_HOLE(db->db_blkptr));
+			dmu_buf_unlock_parent(db, type, FTAG);
+			if (dirty) {
 				dmu_buf_will_dirty(&db->db, tx);
-				rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
 				bzero(db->db.db_data, tail);
 			}
 			dbuf_rele(db, FTAG);
@@ -2120,7 +2136,7 @@
 
 	/* If the range did not include a full block, we are done */
 	if (len == 0)
-		goto out;
+		return;
 
 	ASSERT(IS_P2ALIGNED(off, blksz));
 	ASSERT(trunc || IS_P2ALIGNED(len, blksz));
@@ -2150,6 +2166,7 @@
 	 *    amount of space if we copy the freed BPs into deadlists.
 	 */
 	if (dn->dn_nlevels > 1) {
+		rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
 		uint64_t first, last;
 
 		first = blkid >> epbs;
@@ -2194,6 +2211,7 @@
 
 			dnode_dirty_l1(dn, i, tx);
 		}
+		rw_exit(&dn->dn_struct_rwlock);
 	}
 
 done:
@@ -2215,9 +2233,6 @@
 
 	dbuf_free_range(dn, blkid, blkid + nblks - 1, tx);
 	dnode_setdirty(dn, tx);
-out:
-
-	rw_exit(&dn->dn_struct_rwlock);
 }
 
 static boolean_t
@@ -2329,6 +2344,8 @@
 	dprintf("probing object %llu offset %llx level %d of %u\n",
 	    dn->dn_object, *offset, lvl, dn->dn_phys->dn_nlevels);
 
+	ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
+
 	hole = ((flags & DNODE_FIND_HOLE) != 0);
 	inc = (flags & DNODE_FIND_BACKWARDS) ? -1 : 1;
 	ASSERT(txg == 0 || !hole);
@@ -2361,9 +2378,9 @@
 			return (error);
 		}
 		data = db->db.db_data;
+		rw_enter(&db->db_rwlock, RW_READER);
 	}
 
-
 	if (db != NULL && txg != 0 && (db->db_blkptr == NULL ||
 	    db->db_blkptr->blk_birth <= txg ||
 	    BP_IS_HOLE(db->db_blkptr))) {
@@ -2423,8 +2440,10 @@
 			error = SET_ERROR(ESRCH);
 	}
 
-	if (db)
+	if (db != NULL) {
+		rw_exit(&db->db_rwlock);
 		dbuf_rele(db, FTAG);
+	}
 
 	return (error);
 }
--- a/usr/src/uts/common/fs/zfs/dnode_sync.c	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/dnode_sync.c	Tue Nov 17 10:47:03 2020 -0600
@@ -52,7 +52,6 @@
 
 	/* this dnode can't be paged out because it's dirty */
 	ASSERT(dn->dn_phys->dn_type != DMU_OT_NONE);
-	ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));
 	ASSERT(new_level > 1 && dn->dn_phys->dn_nlevels > 0);
 
 	db = dbuf_hold_level(dn, dn->dn_phys->dn_nlevels, 0, FTAG);
@@ -62,8 +61,24 @@
 	dprintf("os=%p obj=%llu, increase to %d\n", dn->dn_objset,
 	    dn->dn_object, dn->dn_phys->dn_nlevels);
 
+	/*
+	 * Lock ordering requires that we hold the children's db_mutexes (by
+	 * calling dbuf_find()) before holding the parent's db_rwlock.  The lock
+	 * order is imposed by dbuf_read's steps of "grab the lock to protect
+	 * db_parent, get db_parent, hold db_parent's db_rwlock".
+	 */
+	dmu_buf_impl_t *children[DN_MAX_NBLKPTR];
+	ASSERT3U(nblkptr, <=, DN_MAX_NBLKPTR);
+	for (i = 0; i < nblkptr; i++) {
+		children[i] =
+		    dbuf_find(dn->dn_objset, dn->dn_object, old_toplvl, i);
+	}
+
 	/* transfer dnode's block pointers to new indirect block */
 	(void) dbuf_read(db, NULL, DB_RF_MUST_SUCCEED|DB_RF_HAVESTRUCT);
+	if (dn->dn_dbuf != NULL)
+		rw_enter(&dn->dn_dbuf->db_rwlock, RW_WRITER);
+	rw_enter(&db->db_rwlock, RW_WRITER);
 	ASSERT(db->db.db_data);
 	ASSERT(arc_released(db->db_buf));
 	ASSERT3U(sizeof (blkptr_t) * nblkptr, <=, db->db.db_size);
@@ -73,8 +88,7 @@
 
 	/* set dbuf's parent pointers to new indirect buf */
 	for (i = 0; i < nblkptr; i++) {
-		dmu_buf_impl_t *child =
-		    dbuf_find(dn->dn_objset, dn->dn_object, old_toplvl, i);
+		dmu_buf_impl_t *child = children[i];
 
 		if (child == NULL)
 			continue;
@@ -107,6 +121,10 @@
 
 	bzero(dn->dn_phys->dn_blkptr, sizeof (blkptr_t) * nblkptr);
 
+	rw_exit(&db->db_rwlock);
+	if (dn->dn_dbuf != NULL)
+		rw_exit(&dn->dn_dbuf->db_rwlock);
+
 	dbuf_rele(db, FTAG);
 
 	rw_exit(&dn->dn_struct_rwlock);
@@ -183,7 +201,7 @@
 		ASSERT(db->db_level == 1);
 
 		rw_enter(&dn->dn_struct_rwlock, RW_READER);
-		err = dbuf_hold_impl(dn, db->db_level-1,
+		err = dbuf_hold_impl(dn, db->db_level - 1,
 		    (db->db_blkid << epbs) + i, TRUE, FALSE, FTAG, &child);
 		rw_exit(&dn->dn_struct_rwlock);
 		if (err == ENOENT)
@@ -281,7 +299,9 @@
 	 * ancestor of the first or last block to be freed.  The first and
 	 * last L1 indirect blocks are always dirtied by dnode_free_range().
 	 */
+	db_lock_type_t dblt = dmu_buf_lock_parent(db, RW_READER, FTAG);
 	VERIFY(BP_GET_FILL(db->db_blkptr) == 0 || db->db_dirtycnt > 0);
+	dmu_buf_unlock_parent(db, dblt, FTAG);
 
 	dbuf_release_bp(db);
 	bp = db->db.db_data;
@@ -307,7 +327,9 @@
 
 	if (db->db_level == 1) {
 		FREE_VERIFY(db, start, end, tx);
-		free_blocks(dn, bp, end-start+1, tx);
+		rw_enter(&db->db_rwlock, RW_WRITER);
+		free_blocks(dn, bp, end - start + 1, tx);
+		rw_exit(&db->db_rwlock);
 	} else {
 		for (uint64_t id = start; id <= end; id++, bp++) {
 			if (BP_IS_HOLE(bp))
@@ -324,10 +346,12 @@
 	}
 
 	if (free_indirects) {
+		rw_enter(&db->db_rwlock, RW_WRITER);
 		for (i = 0, bp = db->db.db_data; i < 1 << epbs; i++, bp++)
 			ASSERT(BP_IS_HOLE(bp));
 		bzero(db->db.db_data, db->db.db_size);
 		free_blocks(dn, db->db_blkptr, 1, tx);
+		rw_exit(&db->db_rwlock);
 	}
 
 	DB_DNODE_EXIT(db);
@@ -379,7 +403,6 @@
 			VERIFY0(dbuf_hold_impl(dn, dnlevel - 1, i,
 			    TRUE, FALSE, FTAG, &db));
 			rw_exit(&dn->dn_struct_rwlock);
-
 			free_children(db, blkid, nblks, free_indirects, tx);
 			dbuf_rele(db, FTAG);
 		}
--- a/usr/src/uts/common/fs/zfs/sys/dbuf.h	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/sys/dbuf.h	Tue Nov 17 10:47:03 2020 -0600
@@ -108,6 +108,12 @@
 	DR_OVERRIDDEN
 } override_states_t;
 
+typedef enum db_lock_type {
+	DLT_NONE,
+	DLT_PARENT,
+	DLT_OBJSET
+} db_lock_type_t;
+
 typedef struct dbuf_dirty_record {
 	/* link on our parents dirty list */
 	list_node_t dr_dirty_node;
@@ -217,6 +223,22 @@
 	 */
 	uint8_t db_level;
 
+	/*
+	 * Protects db_buf's contents if they contain an indirect block or data
+	 * block of the meta-dnode. We use this lock to protect the structure of
+	 * the block tree. This means that when modifying this dbuf's data, we
+	 * grab its rwlock. When modifying its parent's data (including the
+	 * blkptr to this dbuf), we grab the parent's rwlock. The lock ordering
+	 * for this lock is:
+	 * 1) dn_struct_rwlock
+	 * 2) db_rwlock
+	 * We don't currently grab multiple dbufs' db_rwlocks at once.
+	 */
+	krwlock_t db_rwlock;
+
+	/* buffer holding our data */
+	arc_buf_t *db_buf;
+
 	/* db_mtx protects the members below */
 	kmutex_t db_mtx;
 
@@ -232,9 +254,6 @@
 	 */
 	zfs_refcount_t db_holds;
 
-	/* buffer holding our data */
-	arc_buf_t *db_buf;
-
 	kcondvar_t db_changed;
 	dbuf_dirty_record_t *db_data_pending;
 
@@ -336,6 +355,8 @@
 void dbuf_unoverride(dbuf_dirty_record_t *dr);
 void dbuf_sync_list(list_t *list, int level, dmu_tx_t *tx);
 void dbuf_release_bp(dmu_buf_impl_t *db);
+db_lock_type_t dmu_buf_lock_parent(dmu_buf_impl_t *db, krw_t rw, void *tag);
+void dmu_buf_unlock_parent(dmu_buf_impl_t *db, db_lock_type_t type, void *tag);
 
 boolean_t dbuf_can_remap(const dmu_buf_impl_t *buf);
 
--- a/usr/src/uts/common/fs/zfs/sys/dmu_zfetch.h	Fri Dec 04 11:41:39 2020 +0000
+++ b/usr/src/uts/common/fs/zfs/sys/dmu_zfetch.h	Tue Nov 17 10:47:03 2020 -0600
@@ -24,7 +24,7 @@
  */
 
 /*
- * Copyright (c) 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2014, 2017 by Delphix. All rights reserved.
  */
 
 #ifndef	_DMU_ZFETCH_H
@@ -66,7 +66,8 @@
 
 void		dmu_zfetch_init(zfetch_t *, struct dnode *);
 void		dmu_zfetch_fini(zfetch_t *);
-void		dmu_zfetch(zfetch_t *, uint64_t, uint64_t, boolean_t);
+void		dmu_zfetch(zfetch_t *, uint64_t, uint64_t, boolean_t,
+    boolean_t);
 
 
 #ifdef	__cplusplus