在不相关的进程之间共享futex

Question

不相关的流程如何使用futex进行合作？

假设我有不相关的进程,例如,一个是我的模块的apache子进程,另一个是例如后台脚本.

我想使用futex在两者之间建立一个带有互斥的条件变量,以便为用户空间快速代码路径带来好处.

在我看来,存储互斥锁的内存可能位于mmap'd文件中,如果该内存被映射,例如mlock,理论上两个进程可以针对相同的地址发出futex调用.

或者,也许可以使用futex从一个进程传递到另一个进程FUTEX_FD.

代码提交接受低级,高级和动态语言(C,C++,Python等).还必须支持"robust futex"API.

参考文献:

• https://www.kernel.org/doc/Documentation/robust-futexes.txt
• http://locklessinc.com/articles/mutex_cv_futex/

Answer 1

感谢Phillip和Felix M.的指点。

Python用户代码（数据结构文件已存在，用 初始化PTHREAD_PROCESS_SHARED）

with open("/tmp/semaphore", "rb+") as f:
    m = mmap.mmap(f.fileno(), 0)  # default: all file, share, read-write

data = ffi.cast("unsigned long[3]", id(m))[2]  # pointer to mapped area, 64-bit CPython
lock = ffi.cast("pthread_mutex_t *", data)
cond = ffi.cast("pthread_cond_t *", data + 40)

@contextlib.contextmanager
def locked(alock):
    assert not C.pthread_mutex_lock(alock)
    try:
        yield
    finally:
        assert not C.pthread_mutex_unlock(alock)

等待并醒来：

if "wait" in sys.argv:
    with locked(lock):
        assert not C.pthread_cond_wait(cond, lock)

elif "signal" in sys.argv:
    with locked(lock):
        assert not C.pthread_cond_signal(cond)

设置基础PTHREAD_PROCESS_SHARED：

l = ffi.new("pthread_mutexattr_t *")
assert not C.pthread_mutexattr_init(l)
assert not C.pthread_mutexattr_setpshared(l, 1)  # PTHREAD_PROCESS_SHARED
assert not C.pthread_mutex_init(lock, l)
# same for condition variable

挑剔的完整代码:-) https://github.com/dimaqq/pthread_mutexattr_init/blob/master/xsem.py基于http://linux.die.net/man/3/pthread_mutexattr_init