4046 dsl_dataset_t ds_dir->dd_lock is highly contended
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

@@ -839,24 +839,33 @@
 void
 dsl_dir_diduse_space(dsl_dir_t *dd, dd_used_t type,
     int64_t used, int64_t compressed, int64_t uncompressed, dmu_tx_t *tx)
 {
         int64_t accounted_delta;
+
+        /*
+         * dsl_dataset_set_refreservation_sync_impl() calls this with
+         * dd_lock held, so that it can atomically update
+         * ds->ds_reserved and the dsl_dir accounting, so that
+         * dsl_dataset_check_quota() can see dataset and dir accounting
+         * consistently.
+         */
         boolean_t needlock = !MUTEX_HELD(&dd->dd_lock);
 
         ASSERT(dmu_tx_is_syncing(tx));
         ASSERT(type < DD_USED_NUM);
 
+        dmu_buf_will_dirty(dd->dd_dbuf, tx);
+
         if (needlock)
                 mutex_enter(&dd->dd_lock);
         accounted_delta = parent_delta(dd, dd->dd_phys->dd_used_bytes, used);
         ASSERT(used >= 0 || dd->dd_phys->dd_used_bytes >= -used);
         ASSERT(compressed >= 0 ||
             dd->dd_phys->dd_compressed_bytes >= -compressed);
         ASSERT(uncompressed >= 0 ||
             dd->dd_phys->dd_uncompressed_bytes >= -uncompressed);
-        dmu_buf_will_dirty(dd->dd_dbuf, tx);
         dd->dd_phys->dd_used_bytes += used;
         dd->dd_phys->dd_uncompressed_bytes += uncompressed;
         dd->dd_phys->dd_compressed_bytes += compressed;
 
         if (dd->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN) {

@@ -885,29 +894,25 @@
 
 void
 dsl_dir_transfer_space(dsl_dir_t *dd, int64_t delta,
     dd_used_t oldtype, dd_used_t newtype, dmu_tx_t *tx)
 {
-        boolean_t needlock = !MUTEX_HELD(&dd->dd_lock);
-
         ASSERT(dmu_tx_is_syncing(tx));
         ASSERT(oldtype < DD_USED_NUM);
         ASSERT(newtype < DD_USED_NUM);
 
         if (delta == 0 || !(dd->dd_phys->dd_flags & DD_FLAG_USED_BREAKDOWN))
                 return;
 
-        if (needlock)
+        dmu_buf_will_dirty(dd->dd_dbuf, tx);
                 mutex_enter(&dd->dd_lock);
         ASSERT(delta > 0 ?
             dd->dd_phys->dd_used_breakdown[oldtype] >= delta :
             dd->dd_phys->dd_used_breakdown[newtype] >= -delta);
         ASSERT(dd->dd_phys->dd_used_bytes >= ABS(delta));
-        dmu_buf_will_dirty(dd->dd_dbuf, tx);
         dd->dd_phys->dd_used_breakdown[oldtype] -= delta;
         dd->dd_phys->dd_used_breakdown[newtype] += delta;
-        if (needlock)
                 mutex_exit(&dd->dd_lock);
 }
 
 typedef struct dsl_dir_set_qr_arg {
         const char *ddsqra_name;