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);
The lock file should be removed at some point, to avoid filling the temp directory with hundreds of files.
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 !)
Is there a way to remove the lock file at some point without introducing this race condition ?
Sorry if I reply to a dead question:
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);
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.
I actually proved it in the state-machine model, using the following properties:
If P_i has a descriptor locked on the filesystem then no other process is in the critical section.
If P_i is after the stat with the right inode or in the critical section it has the descriptor locked on the filesystem.