fix(fa3): skip scheduler_metadata precompute under DP attention (#24632)

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -224,6 +224,16 @@ class FlashAttentionBackend(AttentionBackend):
             and server_args.disable_radix_cache
         )
 
+        # Skip the FA3 scheduler_metadata precompute (PR #21104) under DP
+        # attention. The precomputed buffer can become inconsistent with the
+        # num_splits the C++ mha_fwd kernel derives from live cache_seqlens
+        # during decode, leading to an OOB read in the split-KV combine kernel
+        # (flash_fwd_combine_launch_template.h:52). Leaving scheduler_metadata
+        # unset uses the existing per-layer metadata path.
+        self._disable_scheduler_metadata_precompute = bool(
+            getattr(server_args, "enable_dp_attention", False)
+        )
+
     def _compute_scheduler_metadata(
         self, batch_size, max_seq_len_k, cache_seqlens, cu_seqlens_q
     ):
@@ -233,6 +243,8 @@ class FlashAttentionBackend(AttentionBackend):
         """
         if self._get_scheduler_metadata is None or self.use_mla:
             return None
+        if self._disable_scheduler_metadata_precompute:
+            return None
         # Always use window_size=(-1, -1) because scheduler_metadata is only
         # consumed by non-SWA layers (SWA layers skip it in forward_decode).
         return self._get_scheduler_metadata(