Às 11:07 de 15/09/21, Peter Zijlstra escreveu:
> From: André Almeida <andrealmeid@xxxxxxxxxxxxx>
>
> Add support to wait on multiple futexes. This is the interface
> implemented by this syscall:
>
> futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes,
> unsigned int flags, struct timespec *timo)
>
> +/**
> + * futex_wait_multiple_setup - Prepare to wait and enqueue multiple futexes
> + * @vs: The futex list to wait on
> + * @count: The size of the list
> + * @awaken: Index of the last awoken futex, if any. Used to notify the
> + * caller that it can return this index to userspace (return parameter)
> + *
> + * Prepare multiple futexes in a single step and enqueue them. This may fail if
> + * the futex list is invalid or if any futex was already awoken. On success the
> + * task is ready to interruptible sleep.
> + *
> + * Return:
> + * - 1 - One of the futexes was awaken by another thread
> + * - 0 - Success
> + * - <0 - -EFAULT, -EWOULDBLOCK or -EINVAL
> + */
> +static int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *awaken)
> +{
> + struct futex_hash_bucket *hb;
> + bool retry = false;
> + int ret, i;
> + u32 uval;
> +
> + /*
> + * Enqueuing multiple futexes is tricky, because we need to enqueue
> + * each futex in the list before dealing with the next one to avoid
> + * deadlocking on the hash bucket. But, before enqueuing, we need to
> + * make sure that current->state is TASK_INTERRUPTIBLE, so we don't
> + * absorb any awake events, which cannot be done before the
> + * get_futex_key of the next key, because it calls get_user_pages,
> + * which can sleep. Thus, we fetch the list of futexes keys in two
> + * steps, by first pinning all the memory keys in the futex key, and
> + * only then we read each key and queue the corresponding futex.
> + *
> + * Private futexes doesn't need to recalculate hash in retry, so skip
> + * get_futex_key() when retrying.
> + */
> +retry:
> + for (i = 0; i < count; i++) {
> + if ((vs[i].w.flags & FUTEX_PRIVATE_FLAG) && retry)
> + continue;
> +
> + ret = get_futex_key(u64_to_user_ptr(vs[i].w.uaddr),
> + !(vs[i].w.flags & FUTEX_PRIVATE_FLAG),
> + &vs[i].q.key, FUTEX_READ);
> +
> + if (unlikely(ret))
> + return ret;
> + }
> +
> + set_current_state(TASK_INTERRUPTIBLE);
> +
> + for (i = 0; i < count; i++) {
> + u32 __user *uaddr = (u32 __user *)(unsigned long)vs[i].w.uaddr;
> + struct futex_q *q = &vs[i].q;
> + u32 val = (u32)vs[i].w.val;
> +
> + hb = futex_q_lock(q);
> + ret = futex_get_value_locked(&uval, uaddr);
> +
> + if (!ret && uval == val) {
> + /*
> + * The bucket lock can't be held while dealing with the
> + * next futex. Queue each futex at this moment so hb can
> + * be unlocked.
> + */
> + futex_queue(q, hb);
> + continue;
> + }
> +
> + futex_q_unlock(hb);
> + __set_current_state(TASK_RUNNING);
> +
> + /*
> + * Even if something went wrong, if we find out that a futex
> + * was awaken, we don't return error and return this index to
> + * userspace
> + */
> + *awaken = unqueue_multiple(vs, i);
> + if (*awaken >= 0)
> + return 1;
> +
> + if (uval != val)
> + return -EWOULDBLOCK;
> +
> + if (ret) {
> + /*
> + * If we need to handle a page fault, we need to do so
> + * without any lock and any enqueued futex (otherwise
> + * we could lose some wakeup). So we do it here, after
> + * undoing all the work done so far. In success, we
> + * retry all the work.
> + */
> + if (get_user(uval, uaddr))
> + return -EFAULT;
> +
> + retry = true;
> + goto retry;
> + }
My bad again: the last two if's should be in the reserve order. If ret
!= 0, the user copy didn't succeed and the value wasn't copied to uval,
thus the comparison (uval != val) should happen only if ret == 0.
> + }
> +
> + return 0;
> +}
