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guard_acquire is a helper function used to implement TSan's __cxa_guard_acquire and pthread_once interceptors. https://reviews.llvm.org/D54664 introduced optional hooks to support cooperative multi-threading. It worked by marking the entire guard_acquire call as a potentially blocking region. In principle, only the contended case needs to be a potentially blocking region. This didn't matter for __cxa_guard_acquire because the compiler emits an inline fast path before calling __cxa_guard_acquire. That is, once we call __cxa_guard_acquire at all, we know we're in the contended case. https://reviews.llvm.org/D107359 then unified the __cxa_guard_acquire and pthread_once interceptors, adding the hooks to pthread_once. However, unlike __cxa_guard_acquire, pthread_once callers are not expected to have an inline fast path. The fast path is inside the function. As a result, TSan unnecessarily calls into the cooperative multi-threading engine on every pthread_once call, despite applications generally expecting pthread_once to be fast after initialization. Fix this by deferring the hooks to the contended case inside guard_acquire.