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Fix TOCTOU race in _maybe_grow_on_pressure

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Multiple concurrent enqueue_work_autoscale callers could all pass the
cooldown check and each add a thread, overshooting max_threads and
bypassing the intended rate limit.

Use compare_exchange_strong on _last_scale_time_usecs so only one
caller wins the cooldown window. Then hold _scaling_mutex for the
thread-count check and _add_thread_locked call so the grow is atomic
with respect to other scaling operations (monitor, set_threads).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
Antonio S. Musumeci 2026-04-09 21:04:37 -04:00
parent 38fee4dd5f
commit 82e7980ec9
1 changed files with 10 additions and 2 deletions
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12
vendored/libfuse/include/thread_pool.hpp
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@ -608,6 +608,15 @@ ThreadPool::_maybe_grow_on_pressure()
if((now - last) < (std::uint64_t)_scaling_config.cooldown_usecs)
return;
// CAS to claim the cooldown window — only one caller wins.
if(!_last_scale_time_usecs.compare_exchange_strong(last,now,
std::memory_order_relaxed))
return;
// Hold _scaling_mutex so the thread-count check and add are atomic
// with respect to other scaling operations.
mutex_lockguard(_scaling_mutex);
std::size_t count;
{
mutex_lockguard(_threads_mutex);
@ -616,8 +625,7 @@ ThreadPool::_maybe_grow_on_pressure()
if(count < _scaling_config.max_threads)
{
add_thread();
_last_scale_time_usecs.store(now,std::memory_order_relaxed);
_add_thread_locked({});
syslog(LOG_DEBUG,
"threadpool (%s): fast-path grow to %zu threads (queue full)",
_name.c_str(),