ZIL: Force writing of open LWB on suspend

Under parallel workloads ZIL may delay writes of open LWBs that
are not full enough.  On suspend we do not expect anything new to
appear since zil_get_commit_list() will not let it pass, only
returning TXG number to wait for.  But I suspect that waiting for
the TXG commit without having the last LWB issued may not wait for
its completion, resulting in panic described in openzfs#17509.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes openzfs#17521

Author: amotin

module/zfs/zil.c

@@ -2901,19 +2901,14 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 
 	ASSERT(MUTEX_HELD(&zilog->zl_issuer_lock));
 
-	/*
-	 * Return if there's nothing to commit before we dirty the fs by
-	 * calling zil_create().
-	 */
-	if (list_is_empty(&zilog->zl_itx_commit_list))
-		return;
-
-	list_create(&nolwb_itxs, sizeof (itx_t), offsetof(itx_t, itx_node));
-	list_create(&nolwb_waiters, sizeof (zil_commit_waiter_t),
-	    offsetof(zil_commit_waiter_t, zcw_node));
-
 	lwb = list_tail(&zilog->zl_lwb_list);
 	if (lwb == NULL) {
+		/*
+		 * Return if there's nothing to commit before we dirty the fs.
+		 */
+		if (list_is_empty(&zilog->zl_itx_commit_list))
+			return;
+
 		lwb = zil_create(zilog);
 	} else {
 		/*
@@ -2941,6 +2936,10 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 		}
 	}
 
+	list_create(&nolwb_itxs, sizeof (itx_t), offsetof(itx_t, itx_node));
+	list_create(&nolwb_waiters, sizeof (zil_commit_waiter_t),
+	    offsetof(zil_commit_waiter_t, zcw_node));
+
 	while ((itx = list_remove_head(&zilog->zl_itx_commit_list)) != NULL) {
 		lr_t *lrc = &itx->itx_lr;
 		uint64_t txg = lrc->lrc_txg;
@@ -3110,7 +3109,8 @@ zil_process_commit_list(zilog_t *zilog, zil_commit_waiter_t *zcw, list_t *ilwbs)
 		 * possible, without significantly impacting the latency
 		 * of each individual itx.
 		 */
-		if (lwb->lwb_state == LWB_STATE_OPENED && !zilog->zl_parallel) {
+		if (lwb->lwb_state == LWB_STATE_OPENED &&
+		    (!zilog->zl_parallel || zilog->zl_suspend > 0)) {
 			zil_burst_done(zilog);
 			list_insert_tail(ilwbs, lwb);
 			lwb = zil_lwb_write_close(zilog, lwb, LWB_STATE_NEW);