Re: [Qemu-devel] [PATCH 19/25] aio: add generic thread-pool facility

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Oct 26, 2012 at 04:05:49PM +0200, Paolo Bonzini wrote:
> +static void event_notifier_ready(EventNotifier *notifier)
> +{
> +    ThreadPoolElement *elem, *next;
> +
> +    event_notifier_test_and_clear(notifier);
> +restart:
> +    QLIST_FOREACH_SAFE(elem, &head, all, next) {
> +        if (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {
> +            continue;
> +        }
> +        if (elem->state == THREAD_DONE) {
> +            trace_thread_pool_complete(elem, elem->common.opaque, elem->ret);
> +        }
> +        if (elem->state == THREAD_DONE && elem->common.cb) {
> +            QLIST_REMOVE(elem, all);
> +            elem->common.cb(elem->common.opaque, elem->ret);

This function didn't take the lock.  First it accessed elem->state and
how it reads elem->ret.  We need to take the lock to ensure both
elem->state and elem->ret have been set - otherwise we could read
elem->ret before the return value was stored.

> +typedef struct ThreadPoolCo {
> +    Coroutine *co;
> +    int ret;
> +} ThreadPoolCo;
> +
> +static void thread_pool_co_cb(void *opaque, int ret)
> +{
> +    ThreadPoolCo *co = opaque;
> +
> +    co->ret = ret;
> +    qemu_coroutine_enter(co->co, NULL);
> +}
> +
> +int coroutine_fn thread_pool_submit_co(ThreadPoolFunc *func, void *arg)
> +{
> +    ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };
> +    assert(qemu_in_coroutine());
> +    thread_pool_submit_aio(func, arg, thread_pool_co_cb, &tpc);
> +    qemu_coroutine_yield();
> +    return tpc.ret;

It's important to understand that the submit_aio, yield, return ret
pattern works because we assume this function was called as part of the
main loop.

If thread_pool_submit_co() was called outside the event loop and global
mutex, then there is a race between the submit_aio and yield steps where
thread_pool_co_cb() is called before this coroutine yields!

I see no reason why this is a problem today but I had to think through
this case when reading the code.

Stefan