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SubjectRe: [PATCH] Futex Asynchronous Interface
On Thu, 6 Jun 2002 09:36:23 -0700 (PDT)
Linus Torvalds <torvalds@transmeta.com> wrote:
> You already have to have a system call to bind the particular fd to the
> futex _anyway_, so do the only sane thing, and allocate the fd _there_,

But Ma, I don't *want* to write a filesystem!

Linus, Al, is there an easier way to do this? I stole this from sockfs,
but I balked at another 50 lines for a proper inode creation, so I just use
the same dentry and inode over and over.

It's still an awful lot of irrelevant code: what can I cut?

/* Everyone needs a dentry and inode */
static struct dentry *futex_dentry;

/* Signal allows caller to avoid the race which would occur if they
set the sigio stuff up afterwards. */
static int futex_fd(struct list_head *head,
struct page *page,
int offset,
int signal)
{
int fd;
struct futex_q *q;
struct file *filp;

if (signal < 0 || signal > _NSIG)
return -EINVAL;

fd = get_unused_fd();
if (fd < 0)
return fd;
filp = get_empty_filp();
if (!filp) {
put_unused_fd(fd);
return -ENFILE;
}
filp->f_op = &futex_fops;
filp->f_dentry = dget(futex_dentry);

if (signal) {
filp->f_owner.pid = current->pid;
filp->f_owner.uid = current->uid;
filp->f_owner.euid = current->euid;
filp->f_owner.signum = signal;
}

q = kmalloc(sizeof(*q), GFP_KERNEL);
if (!q) {
put_unused_fd(fd);
put_filp(filp);
return -ENOMEM;
}

/* Initialize queue structure */
init_waitqueue_head(&q->waiters);
filp->private_data = q;

/* Go for it... */
queue_me(head, q, page, offset, fd, filp);

/* Now we map fd to filp, so userspace can access it */
fd_install(fd, filp);
return 0;
}

/* Oh yeah, makes sense to write a filesystem... */
static struct super_operations futexfs_ops = {
statfs: simple_statfs,
};

static int futexfs_fill_super(struct super_block *sb, void *data, int silent)
{
struct inode *root;
sb->s_blocksize = 1024;
sb->s_blocksize_bits = 10;
sb->s_magic = 0xFBAD1DEA;
sb->s_op = &futexfs_ops;
root = new_inode(sb);
if (!root)
return -ENOMEM;
root->i_mode = S_IFDIR | S_IRUSR | S_IWUSR;
root->i_uid = root->i_gid = 0;
root->i_atime = root->i_mtime = root->i_ctime = CURRENT_TIME;
sb->s_root = d_alloc(NULL, &(const struct qstr) { "futex", 5, 0 });
if (!sb->s_root) {
iput(root);
return -ENOMEM;
}
sb->s_root->d_sb = sb;
sb->s_root->d_parent = sb->s_root;
d_instantiate(sb->s_root, root);
return 0;
}

static struct super_block *
futexfs_get_sb(struct file_system_type *fs_type,
int flags, char *dev_name, void *data)
{
return get_sb_nodev(fs_type, flags, data, futexfs_fill_super);
}

static struct file_system_type futex_fs_type = {
name: "futexfs",
get_sb: futexfs_get_sb,
kill_sb: kill_anon_super,
fs_flags: FS_NOMOUNT,
};

static int __init init(void)
{
unsigned int i;
struct qstr name = { .name = "futex", .len = 5, .hash = 0 };
struct vfsmount *futex_mnt;

register_filesystem(&futex_fs_type);
futex_mnt = kern_mount(&futex_fs_type);
futex_dentry = d_alloc(NULL, &name);
futex_dentry->d_inode = new_inode(futex_mnt->mnt_sb);

for (i = 0; i < ARRAY_SIZE(futex_queues); i++)
INIT_LIST_HEAD(&futex_queues[i]);
return 0;
}
__initcall(init);

--
there are those who do and those who hang on and you don't see too
many doers quoting their contemporaries. -- Larry McVoy
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