羊群（）：没有竞争条件删除锁定的文件？ [英] flock(): removing locked file without race condition?

问题描述

我用羊群（）名为互斥进程间（即某些进程可以决定举行的some_name，它是通过锁定在一个临时目录中名为some_name文件实施了锁：

I'm using flock() for inter-process named mutexes (i.e. some process can decide to hold a lock on "some_name", which is implemented by locking a file named "some_name" in a temp directory:

lockfile = "/tmp/some_name.lock";
fd = open(lockfile, O_CREAT);
flock(fd, LOCK_EX);

do_something();

unlink(lockfile);
flock(fd, LOCK_UN);

锁定文件应该在某个时候被删除，以避免与数百个文件填充临时目录。

The lock file should be removed at some point, to avoid filling the temp directory with hundreds of files.

然而，在这个code明显的竞争条件;例如与进程A，B和C：

However, there is an obvious race condition in this code; example with processes A, B and C:

A opens file
A locks file
B opens file
A unlinks file
A unlocks file
B locks file (B holds a lock on the deleted file)
C opens file (a new file one is created)
C locks file (two processes hold the same named mutex !)

有没有一种方法，以消除在某个时刻锁定文件，而不会引入这种竞争情况？

Is there a way to remove the lock file at some point without introducing this race condition ?

推荐答案

很抱歉，如果我回答一个死的问题：

Sorry if I reply to a dead question:

锁定文件后，打开它的另一个副本，FSTAT两个副本，并检查inode编号，如：

After locking the file, open another copy of it, fstat both copies and check the inode number, like this:

lockfile = "/tmp/some_name.lock";

    while(1) {
        fd = open(lockfile, O_CREAT);
        flock(fd, LOCK_EX);

        fstat(fd, &st0);
        stat(lockfile, &st1);
        if(st0.st_ino == st1.st_ino) break;

        close(fd);
    }

    do_something();

    unlink(lockfile);
    flock(fd, LOCK_UN);

这prevents的竞争条件，因为如果一个程序持有的文件仍然是文件系统上的锁，具有剩余文件所有其他项目将有一个错误的inode编号。

This prevents the race condition, because if a program holds a lock on a file that is still on the file system, every other program that has a leftover file will have a wrong inode number.

其实，我证明了它的状态机模型，使用以下属性：

I actually proved it in the state-machine model, using the following properties:

如果P_i有锁定在文件系统中的描述符，然后没有其他进程处于关键的部分。

If P_i has a descriptor locked on the filesystem then no other process is in the critical section.

如果P_i是用正确的i节点或在临界区统计后，它锁定在文件系统描述符。

If P_i is after the stat with the right inode or in the critical section it has the descriptor locked on the filesystem.

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