dsl_dir: avoid dd_lock during snapshots_changed updates

Avoid holding dd_lock while updating the on-disk
snapshots_changed timestamp.

Both dsl_dir_zapify() and zap_update() may dirty buffers
and recurse into space accounting, which can take dd_lock.
Holding dd_lock across either operation can therefore
preserve the lock-order inversion reported by lockdep.

Only protect the in-memory dd_snap_cmtime update
with dd_lock. Perform the zapify and ZAP update without
dd_lock held, and retry the on-disk write if another updater
advanced dd_snap_cmtime while the write was in progress.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
Co-authored-by: gality369 <gality369@example.com>
Closes #18472

module/zfs/dsl_dir.c

@@ -2304,22 +2304,29 @@ dsl_dir_snap_cmtime_update(dsl_dir_t *dd, dmu_tx_t *tx)
 {
 	dsl_pool_t *dp = dmu_tx_pool(tx);
 	inode_timespec_t t;
+
+	ASSERT(dsl_pool_sync_context(dp));
 	gethrestime(&t);
 
 	mutex_enter(&dd->dd_lock);
 	dd->dd_snap_cmtime = t;
-	if (spa_feature_is_enabled(dp->dp_spa,
-	    SPA_FEATURE_EXTENSIBLE_DATASET)) {
-		objset_t *mos = dd->dd_pool->dp_meta_objset;
-		uint64_t ddobj = dd->dd_object;
-		dsl_dir_zapify(dd, tx);
-		VERIFY0(zap_update(mos, ddobj,
-		    DD_FIELD_SNAPSHOTS_CHANGED,
-		    sizeof (uint64_t),
-		    sizeof (inode_timespec_t) / sizeof (uint64_t),
-		    &t, tx));
-	}
 	mutex_exit(&dd->dd_lock);
+
+	if (!spa_feature_is_enabled(dp->dp_spa,
+	    SPA_FEATURE_EXTENSIBLE_DATASET)) {
+		return;
+	}
+
+	objset_t *mos = dd->dd_pool->dp_meta_objset;
+
+	/*
+	 * dsl_dir_zapify() and zap_update() may dirty buffers and recurse
+	 * into space accounting, so do not call them with dd_lock held.
+	 */
+	dsl_dir_zapify(dd, tx);
+	VERIFY0(zap_update(mos, dd->dd_object, DD_FIELD_SNAPSHOTS_CHANGED,
+	    sizeof (uint64_t),
+	    sizeof (inode_timespec_t) / sizeof (uint64_t), &t, tx));
 }
 
 void