Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions

On Mon, Feb 15, 2021 at 12:23:52PM -0300, André Almeida wrote:

+static int futex_dequeue_multiple(struct futexv_head *futexv, unsigned int nr) +{

• int i, ret = -1;
• for (i = 0; i < nr; i++) {

• spin_lock(&futexv->objects[i].bucket->lock);
  

• if (!list_empty_careful(&futexv->objects[i].list)) {
  

• 	list_del_init_careful(&futexv->objects[i].list);
  

• 	bucket_dec_waiters(futexv->objects[i].bucket);
  

What's with the careful? AFAICT all sites have that bucket->lock.

• } else {
  

• 	ret = i;
  

• }
  

• spin_unlock(&futexv->objects[i].bucket->lock);
  

• }
• return ret;

+}

+static int futex_enqueue(struct futexv_head *futexv, unsigned int nr_futexes,

• 	 int *awakened)
  

+{

• int i, ret;
• u32 uval, *uaddr, val;
• struct futex_bucket *bucket;

+retry:

• set_current_state(TASK_INTERRUPTIBLE);
• for (i = 0; i < nr_futexes; i++) {

• uaddr = (u32 * __user)futexv->objects[i].uaddr;
  

• val = (u32)futexv->objects[i].val;
  

• bucket = futexv->objects[i].bucket;
  

• bucket_inc_waiters(bucket);
  

• spin_lock(&bucket->lock);
  

• ret = futex_get_user(&uval, uaddr);
  

• if (unlikely(ret)) {
  

• 	spin_unlock(&bucket->lock);
  

• 	bucket_dec_waiters(bucket);
  

• 	__set_current_state(TASK_RUNNING);
  

• 	*awakened = futex_dequeue_multiple(futexv, i);
  

• 	if (__get_user(uval, uaddr))
  

• 		return -EFAULT;
  

• 	if (*awakened >= 0)
  

• 		return 1;
  

• 	goto retry;
  

• }
  

• if (uval != val) {
  

• 	spin_unlock(&bucket->lock);
  

• 	bucket_dec_waiters(bucket);
  

• 	__set_current_state(TASK_RUNNING);
  

• 	*awakened = futex_dequeue_multiple(futexv, i);
  

• 	if (*awakened >= 0)
  

• 		return 1;
  

• 	return -EAGAIN;
  

• }
  

• list_add_tail(&futexv->objects[i].list, &bucket->list);
  

and here

• spin_unlock(&bucket->lock);
  

• }
• return 0;

+}

+static void futex_mark_wake(struct futex_waiter *waiter,

• 	    struct futex_bucket *bucket,
  

• 	    struct wake_q_head *wake_q)
  

+{

• struct task_struct *task;
• struct futexv_head *parent = futex_get_parent((uintptr_t)waiter,

• 				      waiter->index);
  

lockdep_assert_held(&bucket->lock);

• parent->hint = true;
• task = parent->task;
• get_task_struct(task);
• list_del_init_careful(&waiter->list);

and here

• wake_q_add_safe(wake_q, task);
• bucket_dec_waiters(bucket);

+}