Karel Zak <kzak@xxxxxxxxxx> writes:
> On Fri, Apr 08, 2016 at 12:59:29AM +0300, Yuriy M. Kaminskiy wrote:
>> Probably, there are better solutions, but I prefer KISS.
>
>> From fcb97ddcbc336bc8860828c47cdaf21c7b1ca655 Mon Sep 17 00:00:00 2001
>> From: "Yuriy M. Kaminskiy" <yumkam@xxxxxxxxx>
>> Date: Fri, 8 Apr 2016 00:38:56 +0300
>> Subject: [PATCH] fsck: fix racing between unlock/unlink and open
>>
>> Process A Process B Process C
>> open()
>> [creates file]
>> lock()
>> [succeed]
>> open()
>> [open existing]
>> lock()...
>> running()
>> close()
>> [...succeed]
>> unlink()
>> running()
>> open()
>> [creates file] {BAD!}
>> lock()
>> [succeed] {BAD!}
>> running() {BAD!}
>> close()
>>
>
> It would be better to check if the file still exist on the filesystem
> with the same inode number, if not then re-open. It means that on race
> you will create a new lock file. (You have to unlock after unlink.)
> See http://stackoverflow.com/questions/17708885/flock-removing-locked-file-without-race-condition
>
> Simple example:
>
> #include <stdio.h>
> #include <sys/file.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <errno.h>
>
> int main(int argc, char **argv)
> {
> const char *lockpath = "test.lock";
> int fd;
> struct stat st_lock, st_file;
>
> while (1) {
> fd = open(lockpath, O_RDONLY|O_CREAT|O_CLOEXEC,
> S_IWUSR|S_IRUSR|S_IRGRP|S_IROTH);
> fstat(fd, &st_lock);
> flock(fd, LOCK_EX);
>
> errno = 0;
> if (stat(lockpath, &st_file) == 0 && st_file.st_ino == st_lock.st_ino)
> break;
Requires stable inode numbers (well, probably acceptable assumption for {/var,}/run).
> if (errno)
> fprintf(stderr, "%d: stat failed: %m\n", getpid());
> else
> fprintf(stderr, "%d: ignore %lu (fs has:%lu)\n", getpid(), st_lock.st_ino, st_file.st_ino);
> close(fd);
> }
>
> fprintf(stderr, "%d: -->locked ino: %lu\n", getpid(), st_lock.st_ino);
> sleep(10);
> unlink(lockpath);
> close(fd);
> return 0;
> }
>
>
> See untested fsck patch below. Comments?
I still prefer KISS. (And I bet it will eat more resources than just
leaving those empty files around.)
(But either way, bug will be fixed, so I do as you like).
[...]
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