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Subject[PATCH] futex pthreads adaptation and cleanup
Date
Linus, please apply.

Name: Futex Update
Author: Rusty Russell
Section: Misc
Status: Beta

D: This changes futex semantics to a simple "sleep if this address
D: equals this value" interface, which is more convenient for building
D: other primitives. It also adds a timeout value.
D:
D: Example library can be found at:
D: http://www.kernel.org/pub/linux/kernel/people/rusty/futex-2.0.tar.gz

diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.13/include/linux/futex.h working-2.5.13-futexfix/include/linux/futex.h
--- linux-2.5.13/include/linux/futex.h Thu Mar 21 14:14:54 2002
+++ working-2.5.13-futexfix/include/linux/futex.h Mon May 6 11:54:12 2002
@@ -2,7 +2,7 @@
#define _LINUX_FUTEX_H

/* Second argument to futex syscall */
-#define FUTEX_UP (0)
-#define FUTEX_DOWN (1)
+#define FUTEX_WAIT (0)
+#define FUTEX_WAKE (1)

#endif
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.13/kernel/futex.c working-2.5.13-futexfix/kernel/futex.c
--- linux-2.5.13/kernel/futex.c Thu Mar 21 14:14:56 2002
+++ working-2.5.13-futexfix/kernel/futex.c Mon May 6 11:59:59 2002
@@ -32,7 +32,8 @@
#include <linux/fs.h>
#include <linux/futex.h>
#include <linux/highmem.h>
-#include <asm/atomic.h>
+#include <linux/time.h>
+#include <asm/uaccess.h>

/* These mutexes are a very simple counter: the winner is the one who
decrements from 1 to 0. The counter starts at 1 when the lock is
@@ -68,22 +69,27 @@
return &futex_queues[hash_long(h, FUTEX_HASHBITS)];
}

-static inline void wake_one_waiter(struct list_head *head,
- struct page *page,
- unsigned int offset)
+static int futex_wake(struct list_head *head,
+ struct page *page,
+ unsigned int offset,
+ int num)
{
- struct list_head *i;
+ struct list_head *i, *next;
+ int num_woken = 0;

spin_lock(&futex_lock);
- list_for_each(i, head) {
+ list_for_each_safe(i, next, head) {
struct futex_q *this = list_entry(i, struct futex_q, list);

if (this->page == page && this->offset == offset) {
+ list_del_init(i);
wake_up_process(this->task);
- break;
+ num_woken++;
+ if (num_woken >= num) break;
}
}
spin_unlock(&futex_lock);
+ return num_woken;
}

/* Add at end to avoid starvation */
@@ -101,11 +107,17 @@
spin_unlock(&futex_lock);
}

-static inline void unqueue_me(struct futex_q *q)
+/* Return 1 if we were still queued (ie. 0 means we were woken) */
+static inline int unqueue_me(struct futex_q *q)
{
+ int ret = 0;
spin_lock(&futex_lock);
- list_del(&q->list);
+ if (!list_empty(&q->list)) {
+ list_del(&q->list);
+ ret = 1;
+ }
spin_unlock(&futex_lock);
+ return ret;
}

/* Get kernel address of the user page and pin it. */
@@ -129,74 +141,65 @@
return page;
}

-/* Try to decrement the user count to zero. */
-static int decrement_to_zero(struct page *page, unsigned int offset)
-{
- atomic_t *count;
- int ret = 0;
-
- count = kmap(page) + offset;
- /* If we take the semaphore from 1 to 0, it's ours. If it's
- zero, decrement anyway, to indicate we are waiting. If
- it's negative, don't decrement so we don't wrap... */
- if (atomic_read(count) >= 0 && atomic_dec_and_test(count))
- ret = 1;
- kunmap(page);
- return ret;
-}
-
-/* Simplified from arch/ppc/kernel/semaphore.c: Paul M. is a genius. */
-static int futex_down(struct list_head *head, struct page *page, int offset)
+static int futex_wait(struct list_head *head,
+ struct page *page,
+ int offset,
+ int val,
+ int *uaddr,
+ unsigned long time)
{
- int retval = 0;
+ int curval;
struct futex_q q;
+ int ret = 0;

- current->state = TASK_INTERRUPTIBLE;
+ set_current_state(TASK_INTERRUPTIBLE);
queue_me(head, &q, page, offset);

- while (!decrement_to_zero(page, offset)) {
- if (signal_pending(current)) {
- retval = -EINTR;
- break;
- }
- schedule();
- current->state = TASK_INTERRUPTIBLE;
- }
- current->state = TASK_RUNNING;
- unqueue_me(&q);
- /* If we were signalled, we might have just been woken: we
- must wake another one. Otherwise we need to wake someone
- else (if they are waiting) so they drop the count below 0,
- and when we "up" in userspace, we know there is a
- waiter. */
- wake_one_waiter(head, page, offset);
- return retval;
-}
-
-static int futex_up(struct list_head *head, struct page *page, int offset)
-{
- atomic_t *count;
+ /* Page is pinned, can't fail */
+ if (get_user(curval, uaddr) != 0)
+ BUG();

- count = kmap(page) + offset;
- atomic_set(count, 1);
- smp_wmb();
- kunmap(page);
- wake_one_waiter(head, page, offset);
- return 0;
+ if (curval != val) {
+ ret = -EWOULDBLOCK;
+ set_current_state(TASK_RUNNING);
+ goto out;
+ }
+ time = schedule_timeout(time);
+ if (time == 0) {
+ ret = -ETIMEDOUT;
+ goto out;
+ }
+ if (signal_pending(current)) {
+ ret = -EINTR;
+ goto out;
+ }
+ out:
+ /* Were we woken up anyway? */
+ if (!unqueue_me(&q))
+ return 0;
+ return ret;
}

-asmlinkage int sys_futex(void *uaddr, int op)
+asmlinkage int sys_futex(void *uaddr, int op, int val, struct timespec *utime)
{
int ret;
unsigned long pos_in_page;
struct list_head *head;
struct page *page;
+ unsigned long time = MAX_SCHEDULE_TIMEOUT;
+
+ if (utime) {
+ struct timespec t;
+ if (copy_from_user(&t, utime, sizeof(t)) != 0)
+ return -EFAULT;
+ time = timespec_to_jiffies(&t) + 1;
+ }

pos_in_page = ((unsigned long)uaddr) % PAGE_SIZE;

/* Must be "naturally" aligned, and not on page boundary. */
- if ((pos_in_page % __alignof__(atomic_t)) != 0
- || pos_in_page + sizeof(atomic_t) > PAGE_SIZE)
+ if ((pos_in_page % __alignof__(int)) != 0
+ || pos_in_page + sizeof(int) > PAGE_SIZE)
return -EINVAL;

/* Simpler if it doesn't vanish underneath us. */
@@ -206,13 +209,12 @@

head = hash_futex(page, pos_in_page);
switch (op) {
- case FUTEX_UP:
- ret = futex_up(head, page, pos_in_page);
+ case FUTEX_WAIT:
+ ret = futex_wait(head, page, pos_in_page, val, uaddr, time);
break;
- case FUTEX_DOWN:
- ret = futex_down(head, page, pos_in_page);
+ case FUTEX_WAKE:
+ ret = futex_wake(head, page, pos_in_page, val);
break;
- /* Add other lock types here... */
default:
ret = -EINVAL;
}
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
-
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