Fix data race in CPU-feature global, not just the dispatch pointers - #5
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Fix data race in CPU-feature global, not just the dispatch pointers#5alexey-milovidov wants to merge 1 commit into
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The previous fix (PR #4) made the per-codec dispatch function pointers (crc32_impl, adler32_impl, ...) atomic, but each dispatcher still calls get_x86_cpu_features() / get_arm_cpu_features(), which lazily initialize the global libdeflate_x86_cpu_features / libdeflate_arm_cpu_features bitmask on the first call. That global was only 'volatile', so a plain load racing with the store in libdeflate_init_*_cpu_features() is undefined behavior and was still flagged by ThreadSanitizer: WARNING: ThreadSanitizer: data race Write ... libdeflate_init_x86_cpu_features cpu_features.c Read ... get_x86_cpu_features cpu_features.h Location is global 'libdeflate_x86_cpu_features' This reproduced as a "Server died" failure when two HTTP connections first compressed a gzip response concurrently under TSan. Access the global with relaxed __atomic_load_n / __atomic_store_n, matching the dispatch-pointer fix. The first-call initialization is a benign race (every thread computes the same bitmask, a pure function of the CPU), and relaxed ordering suffices because no other memory is published through it. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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The previous bump (cb6b5c9) made the per-codec dispatch function pointers
(`crc32_impl`, `adler32_impl`, ...) atomic, but each dispatcher still calls
`get_x86_cpu_features` / `get_arm_cpu_features`, which lazily initialize the
global `libdeflate_x86_cpu_features` / `libdeflate_arm_cpu_features` bitmask on
the first call. That global was only `volatile`, so a plain load racing with
the store in `libdeflate_init_*_cpu_features` is still undefined behavior and
was flagged by ThreadSanitizer:
WARNING: ThreadSanitizer: data race
Write ... libdeflate_init_x86_cpu_features cpu_features.c:210
Read ... get_x86_cpu_features cpu_features.h:62
Location is global 'libdeflate_x86_cpu_features'
It reproduced as a "Server died" failure under TSan when two HTTP connections
first compressed a gzip error response concurrently (e.g.
`test_profile_max_sessions_for_user` on the amd_tsan integration shard).
The global is now accessed with relaxed `__atomic_load_n` / `__atomic_store_n`,
matching the dispatch-pointer fix. Fork commit:
ClickHouse/libdeflate@ec0718b
Related: ClickHouse/libdeflate#5
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Follow-up to #4. That PR made the per-codec dispatch function pointers atomic, but each dispatcher still calls
get_x86_cpu_features/get_arm_cpu_features, which lazily initialize the globallibdeflate_x86_cpu_features/libdeflate_arm_cpu_featuresbitmask on the first call. That global was onlyvolatile, so a plain load racing with the store inlibdeflate_init_*_cpu_featuresis undefined behavior and was still flagged by ThreadSanitizer:It reproduced as a "Server died" failure in ClickHouse when two HTTP connections first compressed a gzip response concurrently under TSan (ClickHouse/ClickHouse#108074).
Access the global with relaxed
__atomic_load_n/__atomic_store_n, matching the dispatch-pointer fix. Both x86 and ARM. All bundled tests pass.🤖 Generated with Claude Code