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SubjectRe: Futex queue_me/get_user ordering
On Thu, Nov 18, 2004 at 02:47:26PM -0500, Jakub Jelinek wrote:
> The scenario described in futex_wait-fix.patch IMHO can happen even
> if all calls to pthread_cond_signal are done with mutex held around it, i.e.
> A B X Y
> pthread_mutex_lock (&mtx);
> pthread_cond_wait (&cv, &mtx);
> - mtx release *)
> total++ [1/0/0] (0) {}
> pthread_mutex_lock (&mtx);
> pthread_cond_signal (&cv);
> - wake++ [1/1/0] (1) {}
> FUTEX_WAKE, 1 (returns, nothing is queued)
> pthread_mutex_unlock (&mtx);
> pthread_mutex_lock (&mtx);
> pthread_cond_wait (&cv, &mtx);
> - mtx release *)
> total++ [2/1/0] (1) {}
> FUTEX_WAIT, 0
> queue_me [2/1/0] (1) {A}
> 0 != 1
> FUTEX_WAIT, 1
> queue_me [2/1/0] (1) {A,B}
> 1 == 1
> pthread_mutex_lock (&mtx);
> pthread_cond_signal (&cv);
> - wake++ [2/2/0] (2) {A,B}
> FUTEX_WAKE, 1 (unqueues incorrectly A)
> [2/2/0] (2) {B}
> pthread_mutex_unlock (&mtx);
> try to dequeue but already dequeued
> would normally return EWOULDBLOCK here
> but as unqueue_me failed, returns 0
> woken++ [2/2/1] (2) {B}
> schedule_timeout (forever)
> - mtx reacquire
> pthread_cond_wait returns
> pthread_mutex_unlock (&mtx);
>
> -------------------
> the code would like to say pthread_mutex_unlock (&mtx);
> and pthread_exit here, but never reaches there.
...

http://www.ussg.iu.edu/hypermail/linux/kernel/0411.2/0953.html

Your argument in November was that you don't want to slow down the
kernel and that userland must be able to cope with the
non-atomicity of futex syscall.

But with the recent changes to futex.c I think kernel can ensure
atomicity for free.

With get_futex_value_locked doing the user access in_atomic () and
repeating if that failed, I think it would be just a matter of
something as in the patch below (totally untested though).
It would simplify requeue implementation (getting rid of the nqueued
field), as well as never enqueue a futex in futex_wait until
the *uaddr == val uaccess check has shown it should be enqueued.
And I don't think the kernel will be any slower because of that,
in the common case where get_futex_value_locked does not cause
a mm fault (userland typically accessed that memory a few cycles before
the syscall), the futex_wait change is just about doing first half of
queue_me before the user access and second half after it.

--- linux-2.6.11/kernel/futex.c.jj 2005-03-17 04:42:29.000000000 -0500
+++ linux-2.6.11/kernel/futex.c 2005-03-17 05:13:45.000000000 -0500
@@ -97,7 +97,6 @@ struct futex_q {
*/
struct futex_hash_bucket {
spinlock_t lock;
- unsigned int nqueued;
struct list_head chain;
};

@@ -265,7 +264,6 @@ static inline int get_futex_value_locked
inc_preempt_count();
ret = __copy_from_user_inatomic(dest, from, sizeof(int));
dec_preempt_count();
- preempt_check_resched();

return ret ? -EFAULT : 0;
}
@@ -339,7 +337,6 @@ static int futex_requeue(unsigned long u
struct list_head *head1;
struct futex_q *this, *next;
int ret, drop_count = 0;
- unsigned int nqueued;

retry:
down_read(&current->mm->mmap_sem);
@@ -354,23 +351,24 @@ static int futex_requeue(unsigned long u
bh1 = hash_futex(&key1);
bh2 = hash_futex(&key2);

- nqueued = bh1->nqueued;
+ if (bh1 < bh2)
+ spin_lock(&bh1->lock);
+ spin_lock(&bh2->lock);
+ if (bh1 > bh2)
+ spin_lock(&bh1->lock);
+
if (likely(valp != NULL)) {
int curval;

- /* In order to avoid doing get_user while
- holding bh1->lock and bh2->lock, nqueued
- (monotonically increasing field) must be first
- read, then *uaddr1 fetched from userland and
- after acquiring lock nqueued field compared with
- the stored value. The smp_mb () below
- makes sure that bh1->nqueued is read from memory
- before *uaddr1. */
- smp_mb();
-
ret = get_futex_value_locked(&curval, (int __user *)uaddr1);

if (unlikely(ret)) {
+ spin_unlock(&bh1->lock);
+ if (bh1 != bh2)
+ spin_unlock(&bh2->lock);
+
+ preempt_check_resched();
+
/* If we would have faulted, release mmap_sem, fault
* it in and start all over again.
*/
@@ -385,21 +383,10 @@ static int futex_requeue(unsigned long u
}
if (curval != *valp) {
ret = -EAGAIN;
- goto out;
+ goto out_unlock;
}
}

- if (bh1 < bh2)
- spin_lock(&bh1->lock);
- spin_lock(&bh2->lock);
- if (bh1 > bh2)
- spin_lock(&bh1->lock);
-
- if (unlikely(nqueued != bh1->nqueued && valp != NULL)) {
- ret = -EAGAIN;
- goto out_unlock;
- }
-
head1 = &bh1->chain;
list_for_each_entry_safe(this, next, head1, list) {
if (!match_futex (&this->key, &key1))
@@ -435,13 +422,9 @@ out:
return ret;
}

-/*
- * queue_me and unqueue_me must be called as a pair, each
- * exactly once. They are called with the hashed spinlock held.
- */
-
/* The key must be already stored in q->key. */
-static void queue_me(struct futex_q *q, int fd, struct file *filp)
+static inline struct futex_hash_bucket *
+queue_lock(struct futex_q *q, int fd, struct file *filp)
{
struct futex_hash_bucket *bh;

@@ -455,11 +438,35 @@ static void queue_me(struct futex_q *q,
q->lock_ptr = &bh->lock;

spin_lock(&bh->lock);
- bh->nqueued++;
+ return bh;
+}
+
+static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *bh)
+{
list_add_tail(&q->list, &bh->chain);
spin_unlock(&bh->lock);
}

+static inline void
+queue_unlock(struct futex_q *q, struct futex_hash_bucket *bh)
+{
+ spin_unlock(&bh->lock);
+ drop_key_refs(&q->key);
+}
+
+/*
+ * queue_me and unqueue_me must be called as a pair, each
+ * exactly once. They are called with the hashed spinlock held.
+ */
+
+/* The key must be already stored in q->key. */
+static void queue_me(struct futex_q *q, int fd, struct file *filp)
+{
+ struct futex_hash_bucket *bh;
+ bh = queue_lock(q, fd, filp);
+ __queue_me(q, bh);
+}
+
/* Return 1 if we were still queued (ie. 0 means we were woken) */
static int unqueue_me(struct futex_q *q)
{
@@ -503,6 +510,7 @@ static int futex_wait(unsigned long uadd
DECLARE_WAITQUEUE(wait, current);
int ret, curval;
struct futex_q q;
+ struct futex_hash_bucket *bh;

retry:
down_read(&current->mm->mmap_sem);
@@ -511,7 +519,7 @@ static int futex_wait(unsigned long uadd
if (unlikely(ret != 0))
goto out_release_sem;

- queue_me(&q, -1, NULL);
+ bh = queue_lock(&q, -1, NULL);

/*
* Access the page AFTER the futex is queued.
@@ -537,14 +545,15 @@ static int futex_wait(unsigned long uadd
ret = get_futex_value_locked(&curval, (int __user *)uaddr);

if (unlikely(ret)) {
+ queue_unlock(&q, bh);
+
+ preempt_check_resched();
+
/* If we would have faulted, release mmap_sem, fault it in and
* start all over again.
*/
up_read(&current->mm->mmap_sem);

- if (!unqueue_me(&q)) /* There's a chance we got woken already */
- return 0;
-
ret = get_user(curval, (int __user *)uaddr);

if (!ret)
@@ -553,9 +562,15 @@ static int futex_wait(unsigned long uadd
}
if (curval != val) {
ret = -EWOULDBLOCK;
- goto out_unqueue;
+ queue_unlock(&q, bh);
+ preempt_check_resched();
+ goto out_release_sem;
}

+ /* Only actually queue if *uaddr contained val. */
+ __queue_me(&q, bh);
+ preempt_check_resched();
+
/*
* Now the futex is queued and we have checked the data, we
* don't want to hold mmap_sem while we sleep.

Jakub
-
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