Commit facdecfa authored by Rusty Russell's avatar Rusty Russell Committed by Linus Torvalds

[PATCH] Futex update.

This changes futex semantics to a simple "sleep if this address
equals this value" interface, which is more convenient for building
other primitives.  It also adds a timeout value.

Example library can be found at:
   http://www.kernel.org/pub/linux/kernel/people/rusty/futex-2.0.tar.gz
parent da320c63
...@@ -2,7 +2,7 @@ ...@@ -2,7 +2,7 @@
#define _LINUX_FUTEX_H #define _LINUX_FUTEX_H
/* Second argument to futex syscall */ /* Second argument to futex syscall */
#define FUTEX_UP (0) #define FUTEX_WAIT (0)
#define FUTEX_DOWN (1) #define FUTEX_WAKE (1)
#endif #endif
...@@ -32,7 +32,8 @@ ...@@ -32,7 +32,8 @@
#include <linux/fs.h> #include <linux/fs.h>
#include <linux/futex.h> #include <linux/futex.h>
#include <linux/highmem.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 /* 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 decrements from 1 to 0. The counter starts at 1 when the lock is
...@@ -68,22 +69,27 @@ static inline struct list_head *hash_futex(struct page *page, ...@@ -68,22 +69,27 @@ static inline struct list_head *hash_futex(struct page *page,
return &futex_queues[hash_long(h, FUTEX_HASHBITS)]; return &futex_queues[hash_long(h, FUTEX_HASHBITS)];
} }
static inline void wake_one_waiter(struct list_head *head, static int futex_wake(struct list_head *head,
struct page *page, struct page *page,
unsigned int offset) unsigned int offset,
int num)
{ {
struct list_head *i; struct list_head *i, *next;
int num_woken = 0;
spin_lock(&futex_lock); 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); struct futex_q *this = list_entry(i, struct futex_q, list);
if (this->page == page && this->offset == offset) { if (this->page == page && this->offset == offset) {
list_del_init(i);
wake_up_process(this->task); wake_up_process(this->task);
break; num_woken++;
if (num_woken >= num) break;
} }
} }
spin_unlock(&futex_lock); spin_unlock(&futex_lock);
return num_woken;
} }
/* Add at end to avoid starvation */ /* Add at end to avoid starvation */
...@@ -101,11 +107,17 @@ static inline void queue_me(struct list_head *head, ...@@ -101,11 +107,17 @@ static inline void queue_me(struct list_head *head,
spin_unlock(&futex_lock); 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); spin_lock(&futex_lock);
list_del(&q->list); if (!list_empty(&q->list)) {
list_del(&q->list);
ret = 1;
}
spin_unlock(&futex_lock); spin_unlock(&futex_lock);
return ret;
} }
/* Get kernel address of the user page and pin it. */ /* Get kernel address of the user page and pin it. */
...@@ -129,74 +141,65 @@ static struct page *pin_page(unsigned long page_start) ...@@ -129,74 +141,65 @@ static struct page *pin_page(unsigned long page_start)
return page; return page;
} }
/* Try to decrement the user count to zero. */ static int futex_wait(struct list_head *head,
static int decrement_to_zero(struct page *page, unsigned int offset) struct page *page,
{ int offset,
atomic_t *count; int val,
int ret = 0; int *uaddr,
unsigned long time)
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)
{ {
int retval = 0; int curval;
struct futex_q q; struct futex_q q;
int ret = 0;
current->state = TASK_INTERRUPTIBLE; set_current_state(TASK_INTERRUPTIBLE);
queue_me(head, &q, page, offset); queue_me(head, &q, page, offset);
while (!decrement_to_zero(page, offset)) { /* Page is pinned, can't fail */
if (signal_pending(current)) { if (get_user(curval, uaddr) != 0)
retval = -EINTR; BUG();
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) if (curval != val) {
{ ret = -EWOULDBLOCK;
atomic_t *count; set_current_state(TASK_RUNNING);
goto out;
count = kmap(page) + offset; }
atomic_set(count, 1); time = schedule_timeout(time);
smp_wmb(); if (time == 0) {
kunmap(page); ret = -ETIMEDOUT;
wake_one_waiter(head, page, offset); goto out;
return 0; }
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; int ret;
unsigned long pos_in_page; unsigned long pos_in_page;
struct list_head *head; struct list_head *head;
struct page *page; 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; pos_in_page = ((unsigned long)uaddr) % PAGE_SIZE;
/* Must be "naturally" aligned, and not on page boundary. */ /* Must be "naturally" aligned, and not on page boundary. */
if ((pos_in_page % __alignof__(atomic_t)) != 0 if ((pos_in_page % __alignof__(int)) != 0
|| pos_in_page + sizeof(atomic_t) > PAGE_SIZE) || pos_in_page + sizeof(int) > PAGE_SIZE)
return -EINVAL; return -EINVAL;
/* Simpler if it doesn't vanish underneath us. */ /* Simpler if it doesn't vanish underneath us. */
...@@ -206,13 +209,12 @@ asmlinkage int sys_futex(void *uaddr, int op) ...@@ -206,13 +209,12 @@ asmlinkage int sys_futex(void *uaddr, int op)
head = hash_futex(page, pos_in_page); head = hash_futex(page, pos_in_page);
switch (op) { switch (op) {
case FUTEX_UP: case FUTEX_WAIT:
ret = futex_up(head, page, pos_in_page); ret = futex_wait(head, page, pos_in_page, val, uaddr, time);
break; break;
case FUTEX_DOWN: case FUTEX_WAKE:
ret = futex_down(head, page, pos_in_page); ret = futex_wake(head, page, pos_in_page, val);
break; break;
/* Add other lock types here... */
default: default:
ret = -EINVAL; ret = -EINVAL;
} }
......
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