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[Bugfix] Fix memory inconsistency in cross-process shared memory #32022

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Merged
njhill merged 2 commits into from
Feb 10, 2026
+6 −0
Merged

[Bugfix] Fix memory inconsistency in cross-process shared memory #32022

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5c10afc
[Bugfix] Fix memory inconsistency in cross-process shared memory
slippersss Jan 9, 2026
1955401
Merge branch 'main' into bugfix
njhill Feb 10, 2026
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6 changes: 6 additions & 0 deletions vllm/distributed/device_communicators/shm_broadcast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -488,6 +488,12 @@ def acquire_write(self, timeout: float | None = None):
for i in range(1, self.buffer.n_reader + 1):
# set read flag to 0, meaning it is not read yet
metadata_buffer[i] = 0
# Memory fence here ensures the order of the buffer and flag
# writes. This guarantees that when `metadata_buffer[0] = 1` is
# visible to readers, `buf` can be completely ready. Without
# this, some CPU architectures with weak ordering may incur
# memory inconsistency.
memory_fence()
# mark the block as written
metadata_buffer[0] = 1
# Memory fence ensures the write is visible to readers on other cores
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