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src/stream/src/executor/backfill/snapshot_backfill.rs

@@ -463,8 +463,14 @@ impl<'a> UpstreamBuffer<'a, StateOfConsumingLogStore> {
     async fn next_barrier(&mut self) -> StreamExecutorResult<Option<DispatcherBarrier>> {
         Ok(
             if let Some(barrier) = self.state.upstream_pending_barriers.pop_back() {
-                // only update the `max_pending_barrier_num` on checkpoint barrier to avoid deadlock caused by
-                // downstream waiting on a checkpoint barrier to bump up.
+                // Only update the `max_pending_barrier_num` on checkpoint barrier to avoid deadlock.
+                //
+                // After updating and decreasing `max_pending_barrier_num`, we won't poll upstream until
+                // the downstream fetches the next barrier, and this causes back pressure to upstream.
+                // However, downstream will be blocked at `try_wait_epoch` on a non-checkpoint epoch on upstream,
+                // and `try_wait_epoch` can only be unblocked when the next checkpoint epoch is processed. But since
+                // the upstream is blocked by back pressure, it cannot process the next checkpoint epoch, which
+                // causes deadlock.
                 if barrier.kind.is_checkpoint() {
                     self.state.max_pending_barrier_num = min(
                         self.state.upstream_pending_barriers.len(),