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[Bugfix] Sync waves before the RMSNorm/LayerNorm quant kernels reuse their reduction slot #1235

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jhinpan wants to merge 1 commit into from
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[Bugfix] Sync waves before the RMSNorm/LayerNorm quant kernels reuse their reduction slot #1235

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4 changes: 4 additions & 0 deletions kernels/norm/layernorm_kernel.py
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Original file line number Diff line number Diff line change
Expand Up @@ -877,6 +877,8 @@ def block_reduce_max(val):
lane = tid % WARP_SIZE
wave = tid // WARP_SIZE
w = wave_reduce_max(val)
# Other waves may still be reading the previous block reduction's result from s_sum[0].
gpu.barrier()

@cschenjunlin cschenjunlin Oct 16, 2026

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This PR adds a gpu.barrier() to block_reduce_max in all four quant builders. It fixes the race caused by reusing the same LDS slot for the sum and max reductions.
However, this PR is based on the main branch as of September 29. Commit 6487994 in #848 has already fixed the same issue, and the current main already contains the following code in rmsnorm_kernel.py (lines 1133–1140):

# s_red still holds the sum reduction's result, which every wave
# broadcast-reads on exit. Only this barrier orders that read
# against the store below; without it a late wave reads this max.
gpu.barrier()
w = wave_reduce_max(val)
if lane == 0:
    fx.memref_store(w, s_red, wave)
gpu.barrier()

I used git merge-tree to simulate the merge. This PR merges cleanly with main, and the redundant barrier could be merged into main without noticing. Therefore, I recommend removing one of duplicative gpu.barrier().

if lane == 0:
fx.memref_store(w, s_sum, wave)
gpu.barrier()
Expand Down Expand Up @@ -1226,6 +1228,8 @@ def block_reduce_max(val):
lane = tid % WARP_SIZE
wave = tid // WARP_SIZE
w = wave_reduce_max(val)
# Other waves may still be reading the previous block reduction's result from s_sum[0].
gpu.barrier()
if lane == 0:
fx.memref_store(w, s_sum, wave)
gpu.barrier()
Expand Down
4 changes: 4 additions & 0 deletions kernels/norm/rmsnorm_kernel.py
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -842,6 +842,8 @@ def block_reduce_max(val):
wave = tid // WARP_SIZE

w = wave_reduce_max(val)
# Other waves may still be reading the previous block reduction's result from s_red[0].
gpu.barrier()
if lane == 0:
fx.memref_store(w, s_red, wave)
gpu.barrier()
Expand Down Expand Up @@ -1211,6 +1213,8 @@ def block_reduce_max(val):
wave = tid // WARP_SIZE

w = wave_reduce_max(val)
# Other waves may still be reading the previous block reduction's result from s_red[0].
gpu.barrier()
if lane == 0:
fx.memref_store(w, s_red, wave)
gpu.barrier()
Expand Down
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