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    SubjectRe: Question about pseudo filesystems
    On Wednesday 04 September 2002 20:02, Jamie Lokier wrote:
    > I have a small problem with a module I'm writing. It uses a
    > pseudo-filesystem, rather like futexes and pipes, so that it can hand
    > out special file descriptors on request.
    > Following the examples of pipe.c and futex.c, but specifically for a 2.4
    > kernel, I'm doing this:
    > static DECLARE_FSTYPE (mymod_fs_type, "mymod_fs",
    > mymod_read_super, FS_NOMOUNT);
    > static int __init mymod_init (void)
    > {
    > int err = register_filesystem (&mymod_fs_type);
    > if (err)
    > return err;
    > mymod_mnt = kern_mount (&mymod_fs_type);
    > if (IS_ERR (mymod_mnt)) {
    > unregister_filesystem (&mymod_fs_type);
    > return PTR_ERR (mymod_mnt);
    > }
    > }
    > static void __exit mymod_exit (void)
    > {
    > mntput (mymod_mnt);
    > unregister_filesystem (&mymod_fs_type);
    > }
    > Unfortunately, when I come to _unload_ the module, it can't be unloaded
    > because the kern_mount increments the module reference count.
    > (pipe.c in 2.4 appears to have the same problem, but of course nobody
    > can ever unload it anyway so it doesn't matter).
    > I'm handing out file descriptors rather like futexes from 2.5: they all
    > share the same dentry, which is the root of the filesystem. In my
    > code's case, that dentry is created in `mymod_read_super' (just the same
    > way as 2.4 pipe.c).
    > Somehow, it looks like I need to mount the filesystem when it first
    > generates an fd, and unmount it when the last fd is destroyed -- but is
    > it safe to unmount the filesystem _within_ a release function of an
    > inode on the filesystem?
    > Either that, or I need something else.

    It's a good example of why the module interface is stupidly wrong, and
    __exit needs to be called by the module unloader, returning 0 if it's
    ok to unload. Then your __exit can whatever condition it's interested
    in and, if all is well, do the kern_umount.

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