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Subject[PATCH] Futex Asynchronous Interface
Date
These two patches (requiring the other patches I sent to the list
which can also be found on my kernel.org page) add the ability to tie
a futex to a file descriptor, for use with poll/select or SIGIO
(required by NGPT).

The method is: open /dev/futex, use sys_futex(FUTEX_AWAIT) to attach
it to a particular futex, then use select or poll (or set the fd up
for sigio signals, and expect a SIGIO).

You need to use FUTEX_AWAIT again after poll succeeds or SIGIO
(ie. it's oneshot). Calling it while a futex is already outstanding
forgets about the old futex.

The reason for this method is that it's pretty convenient for
programs, and since each one pins a page down, tying that to a struct
file * means we have an implicit limit.

Code below. Feedback welcome.
Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

Name: Waker can unpin page, rather than waiting process
Author: Rusty Russell
Status: Tested in 2.5.20
Depends: Futex/copy-from-user.patch.gz Futex/unpin-page-fix.patch.gz
Depends: Futex/waitq.patch.gz

D: This changes the implementation so that the waker actually unpins
D: the page. This is preparation for the async interface, where the
D: process which registered interest is not in the kernel.


diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.20.19104/kernel/futex.c linux-2.5.20.19104.updated/kernel/futex.c
--- linux-2.5.20.19104/kernel/futex.c Thu Jun 6 17:13:46 2002
+++ linux-2.5.20.19104.updated/kernel/futex.c Thu Jun 6 17:14:30 2002
@@ -98,11 +98,13 @@
if (this->page == page && this->offset == offset) {
list_del_init(i);
tell_waiter(this);
+ unpin_page(this->page);
num_woken++;
if (num_woken >= num) break;
}
}
spin_unlock(&futex_lock);
+ unpin_page(page);
return num_woken;
}

@@ -192,9 +194,10 @@
}
out:
set_current_state(TASK_RUNNING);
- /* Were we woken up anyway? */
+ /* Were we woken up anyway? If so, it unpinned page. */
if (!unqueue_me(&q))
return 0;
+ unpin_page(page);
return ret;
}

@@ -225,6 +228,7 @@
if (IS_ERR(page))
return PTR_ERR(page);

+ /* On success, these routines unpin the pages themselves. */
head = hash_futex(page, pos_in_page);
switch (op) {
case FUTEX_WAIT:
@@ -236,7 +240,8 @@
default:
ret = -EINVAL;
}
- unpin_page(page);
+ if (ret < 0)
+ unpin_page(page);

return ret;
}
Name: Asynchronous interface for futexes
Author: Rusty Russell
Status: Tested on 2.5.20
Depends: Futex/comment-fix.patch.gz Futex/copy-from-user.patch.gz
Depends: Futex/no-write-needed.patch.gz Futex/unpin-page-fix.patch.gz
Depends: Futex/waitq.patch.gz Futex/waker-unpin-page.patch.gz
D: This patch adds a FUTEX_AWAIT and /dev/futex, for attaching futexes
D: to file descriptors, which can be used with poll, select or SIGIO.

diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.20.15557/include/linux/futex.h linux-2.5.20.15557.updated/include/linux/futex.h
--- linux-2.5.20.15557/include/linux/futex.h Sat May 25 14:34:59 2002
+++ linux-2.5.20.15557.updated/include/linux/futex.h Wed Jun 5 22:01:44 2002
@@ -4,5 +4,6 @@
/* Second argument to futex syscall */
#define FUTEX_WAIT (0)
#define FUTEX_WAKE (1)
+#define FUTEX_AWAIT (2)

#endif
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.20.15557/kernel/futex.c linux-2.5.20.15557.updated/kernel/futex.c
--- linux-2.5.20.15557/kernel/futex.c Wed Jun 5 22:01:41 2002
+++ linux-2.5.20.15557.updated/kernel/futex.c Wed Jun 5 22:02:09 2002
@@ -34,6 +34,10 @@
#include <linux/highmem.h>
#include <linux/time.h>
#include <linux/pagemap.h>
+#include <linux/file.h>
+#include <linux/slab.h>
+#include <linux/devfs_fs_kernel.h>
+#include <linux/poll.h>
#include <asm/uaccess.h>

/* Simple "sleep if unchanged" interface. */
@@ -41,11 +45,18 @@
/* FIXME: This may be way too small. --RR */
#define FUTEX_HASHBITS 6

+extern void send_sigio(struct fown_struct *fown, int fd, int band);
+
/* We use this instead of a normal wait_queue_t, so we can wake only
the relevent ones (hashed queues may be shared) */
struct futex_q {
struct list_head list;
wait_queue_head_t waiters;
+
+ /* For AWAIT, sigio sent using these. */
+ int fd;
+ struct file *filp;
+
/* Page struct and offset within it. */
struct page *page;
unsigned int offset;
@@ -54,6 +65,7 @@
/* The key for the hash is the address + index + offset within page */
static struct list_head futex_queues[1<<FUTEX_HASHBITS];
static spinlock_t futex_lock = SPIN_LOCK_UNLOCKED;
+extern struct file_operations futex_fops;

static inline struct list_head *hash_futex(struct page *page,
unsigned long offset)
@@ -73,9 +85,12 @@
page_cache_release(page);
}

+/* Waiter may be sitting in FUTEX_WAIT or poll, or async */
static inline void tell_waiter(struct futex_q *q)
{
wake_up_all(&q->waiters);
+ if (q->fd != -1)
+ send_sigio(&q->filp->f_owner, q->fd, POLL_IN);
}

static int futex_wake(struct list_head *head,
@@ -113,6 +128,7 @@
add_wait_queue(&q->waiters, wait);
q->page = page;
q->offset = offset;
+ q->fd = -1;

spin_lock(&futex_lock);
list_add_tail(&q->list, head);
@@ -196,6 +212,38 @@
return ret;
}

+static int futex_await(struct list_head *head,
+ struct page *page,
+ int offset,
+ int fd)
+{
+ struct file *filp;
+ struct futex_q *q;
+
+ filp = fget(fd);
+ if (!filp || filp->f_op != &futex_fops)
+ return -EBADF;
+ q = filp->private_data;
+
+ spin_lock(&futex_lock);
+ /* Eliminate any old notification, wake any pollers, release page. */
+ if (!list_empty(&q->list)) {
+ list_del(&q->list);
+ wake_up_all(&q->waiters);
+ unpin_page(q->page);
+ }
+
+ q->filp = filp;
+ q->fd = fd;
+ q->page = page;
+ q->offset = offset;
+ list_add_tail(&q->list, head);
+ spin_unlock(&futex_lock);
+ fput(filp);
+
+ return 0;
+}
+
asmlinkage int sys_futex(void *uaddr, int op, int val, struct timespec *utime)
{
int ret;
@@ -229,6 +277,9 @@
case FUTEX_WAIT:
ret = futex_wait(head, page, pos_in_page, val, uaddr, time);
break;
+ case FUTEX_AWAIT:
+ ret = futex_await(head, page, pos_in_page, val);
+ break;
case FUTEX_WAKE:
ret = futex_wake(head, page, pos_in_page, val);
break;
@@ -241,12 +292,68 @@
return ret;
}

+static int futex_open(struct inode *inode, struct file *filp)
+{
+ struct futex_q *q;
+
+ q = kmalloc(sizeof(*q), GFP_KERNEL);
+ if (!q)
+ return -ENOMEM;
+ INIT_LIST_HEAD(&q->list);
+ init_waitqueue_head(&q->waiters);
+
+ filp->private_data = q;
+ return 0;
+}
+
+static int futex_close(struct inode *inode, struct file *filp)
+{
+ struct futex_q *q = filp->private_data;
+
+ spin_lock(&futex_lock);
+ if (!list_empty(&q->list)) {
+ list_del(&q->list);
+ unpin_page(q->page);
+ BUG_ON(waitqueue_active(&q->waiters));
+ }
+ spin_unlock(&futex_lock);
+ kfree(filp->private_data);
+ return 0;
+}
+
+/* You need to do a FUTEX_AWAIT to arm this after each successful poll */
+static unsigned int futex_poll(struct file *filp,
+ struct poll_table_struct *wait)
+{
+ struct futex_q *q = filp->private_data;
+ int ret = 0;
+
+ spin_lock(&futex_lock);
+ if (!list_empty(&q->list))
+ poll_wait(filp, &q->waiters, wait);
+ else
+ ret = POLLIN | POLLRDNORM;
+ spin_unlock(&futex_lock);
+
+ return ret;
+}
+
+static struct file_operations futex_fops = {
+ open: futex_open,
+ release: futex_close,
+ poll: futex_poll,
+};
+
static int __init init(void)
{
+ int futex_major;
unsigned int i;

for (i = 0; i < ARRAY_SIZE(futex_queues); i++)
INIT_LIST_HEAD(&futex_queues[i]);
+ futex_major = devfs_register_chrdev(0, "futex", &futex_fops);
+ devfs_register(NULL, "futex", DEVFS_FL_NONE, futex_major,
+ 0, S_IFCHR | 0666, &futex_fops, NULL);
return 0;
}
__initcall(init);
-
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