Decrease contention on dn_struct_rwlock

Currently, sequential async write workloads spend a lot of time 
contending on the dn_struct_rwlock. This lock is responsible for 
protecting the entire block tree below it; this naturally results 
in some serialization during heavy write workloads. This can be 
resolved by having per-dbuf locking, which will allow multiple 
writers in the same object at the same time.

We introduce a new rwlock, the db_rwlock. This lock is responsible 
for protecting the contents of the dbuf that it is a part of; when 
reading a block pointer from a dbuf, you hold the lock as a reader. 
When writing data to a dbuf, you hold it as a writer. This allows 
multiple threads to write to different parts of a file at the same 
time.

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>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
External-issue: DLPX-52564
External-issue: DLPX-53085
External-issue: DLPX-57384
Closes #8946

include/sys/dbuf.h

@@ -108,6 +108,12 @@ typedef enum override_states {
 	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 @@ typedef struct dmu_buf_impl {
 	 */
 	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 @@ typedef struct dmu_buf_impl {
 	 */
 	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;
 
@@ -335,6 +354,8 @@ void dbuf_destroy(dmu_buf_impl_t *db);
 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);
 
 void dbuf_free_range(struct dnode *dn, uint64_t start, uint64_t end,
     struct dmu_tx *);