bthread::Mutex在pthread模式下跟butil:mutex(即linux下pthread_mutex_lock)有啥区别

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**
bthread::Mutex在线程模式会切换到butex_wait->butex_wait_from_pthread->futex_wait_private

那么问题是此时和普通linux的pthread_mutex_lock会有什么差异么？因为看到butil:mutex还是有封装pthread_mutex_lock,如果没差异，按理都无脑使用bthread::Mutex即可，但是不少brpc源码在锁出一些cpu运算没有切bthread的场景，还是选择了butil:mutex（即pthread_mutex_lock）
这种选择是有什么隐含的考虑么？

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**Describe alternatives you've considered (描述你想到的折衷方案)**

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No files, tests, or entry points are named. Start by identifying the brpc implementations of bthread::Mutex and butil::mutex, then compare their pthread-mode behavior and the contexts where each is used. Done means documenting the behavioral difference and the rationale for choosing one in CPU-bound, non-bthread code paths.

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