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SubjectRe: [PATCH 14/32] aio: Make aio_read_evt() more efficient, convert to hrtimers
On Wed, 26 Dec 2012 17:59:52 -0800
Kent Overstreet <koverstreet@google.com> wrote:

> Previously, aio_read_event() pulled a single completion off the
> ringbuffer at a time, locking and unlocking each time. Changed it to
> pull off as many events as it can at a time, and copy them directly to
> userspace.
>
> This also fixes a bug where if copying the event to userspace failed,
> we'd lose the event.
>
> Also convert it to wait_event_interruptible_hrtimeout(), which
> simplifies it quite a bit.
>
> ...
>
> -static int aio_read_evt(struct kioctx *ioctx, struct io_event *ent)
> +static int aio_read_events_ring(struct kioctx *ctx,
> + struct io_event __user *event, long nr)
> {
> - struct aio_ring_info *info = &ioctx->ring_info;
> + struct aio_ring_info *info = &ctx->ring_info;
> struct aio_ring *ring;
> - unsigned long head;
> - int ret = 0;
> + unsigned head, pos;
> + int ret = 0, copy_ret;
> +
> + if (!mutex_trylock(&info->ring_lock)) {
> + __set_current_state(TASK_RUNNING);
> + mutex_lock(&info->ring_lock);
> + }

You're not big on showing your homework, I see :(

I agree that calling mutex_lock() in state TASK_[UN]INTERRUPTIBLE is at
least poor practice. Assuming this is what the code is trying to do.
But if aio_read_events_ring() is indeed called in state
TASK_[UN]INTERRUPTIBLE then the effect of the above code is to put the
task into an *unknown* state.

IOW, I don't have the foggiest clue what you're trying to do here and
you owe us all a code comment. At least.


> ring = kmap_atomic(info->ring_pages[0]);
> - pr_debug("h%u t%u m%u\n", ring->head, ring->tail, ring->nr);
> + head = ring->head;
> + kunmap_atomic(ring);
> +
> + pr_debug("h%u t%u m%u\n", head, info->tail, info->nr);
>
> - if (ring->head == ring->tail)
> + if (head == info->tail)
> goto out;
>
> - spin_lock(&info->ring_lock);
> -
> - head = ring->head % info->nr;
> - if (head != ring->tail) {
> - struct io_event *evp = aio_ring_event(info, head);
> - *ent = *evp;
> - head = (head + 1) % info->nr;
> - smp_mb(); /* finish reading the event before updatng the head */
> - ring->head = head;
> - ret = 1;
> - put_aio_ring_event(evp);
> + __set_current_state(TASK_RUNNING);
> +
> + while (ret < nr) {
> + unsigned i = (head < info->tail ? info->tail : info->nr) - head;
> + struct io_event *ev;
> + struct page *page;
> +
> + if (head == info->tail)
> + break;
> +
> + i = min_t(int, i, nr - ret);
> + i = min_t(int, i, AIO_EVENTS_PER_PAGE -
> + ((head + AIO_EVENTS_OFFSET) % AIO_EVENTS_PER_PAGE));

min_t() is kernel shorthand for "I screwed up my types". Methinks
`ret' should have long type. Or, better, unsigned (negative makes no
sense). And when a C programmer sees an variable called "i" he thinks
it has type "int", so that guy should be renamed.

Can we please clean all this up?

> + pos = head + AIO_EVENTS_OFFSET;
> + page = info->ring_pages[pos / AIO_EVENTS_PER_PAGE];
> + pos %= AIO_EVENTS_PER_PAGE;
> +
> + ev = kmap(page);
> + copy_ret = copy_to_user(event + ret, ev + pos, sizeof(*ev) * i);
> + kunmap(page);
> +
> + if (unlikely(copy_ret)) {
> + ret = -EFAULT;
> + goto out;
> + }
> +
> + ret += i;
> + head += i;
> + head %= info->nr;
> }
> - spin_unlock(&info->ring_lock);
>
> -out:
> + ring = kmap_atomic(info->ring_pages[0]);
> + ring->head = head;
> kunmap_atomic(ring);
> - pr_debug("%d h%u t%u\n", ret, ring->head, ring->tail);
> +
> + pr_debug("%d h%u t%u\n", ret, head, info->tail);
> +out:
> + mutex_unlock(&info->ring_lock);
> +
> return ret;
> }
>
> ...
>



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