Re: [Qemu-devel] [PATCH 01/16] aio: introduce aio_co_schedule and aio_co_wake

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jan 13, 2017 at 02:17:16PM +0100, Paolo Bonzini wrote:
> +static void co_schedule_bh_cb(void *opaque)
> +{
> +    AioContext *ctx = opaque;
> +    QSLIST_HEAD(, Coroutine) straight, reversed;
> +
> +    QSLIST_MOVE_ATOMIC(&reversed, &ctx->scheduled_coroutines);
> +    QSLIST_INIT(&straight);
> +
> +    while (!QSLIST_EMPTY(&reversed)) {
> +        Coroutine *co = QSLIST_FIRST(&reversed);
> +        QSLIST_REMOVE_HEAD(&reversed, co_scheduled_next);
> +        QSLIST_INSERT_HEAD(&straight, co, co_scheduled_next);
> +    }
> +
> +    while (!QSLIST_EMPTY(&straight)) {
> +        Coroutine *co = QSLIST_FIRST(&straight);
> +        QSLIST_REMOVE_HEAD(&straight, co_scheduled_next);
> +        trace_aio_co_schedule_bh_cb(ctx, co);
> +        qemu_coroutine_enter(co);
> +    }
> +}

ctx->scheduled_coroutines is a specialized CoQueue.  Was there no way to
modify and then use CoQueue instead of open coding it?

> +void aio_co_wake(struct Coroutine *co)
> +{
> +    AioContext *ctx;
> +
> +    /* Read coroutine before co->ctx.  Matches smp_wmb in
> +     * qemu_coroutine_enter.
> +     */
> +    smp_read_barrier_depends();
> +    ctx = atomic_read(&co->ctx);
> +
> +    if (ctx != qemu_get_current_aio_context()) {
> +        aio_co_schedule(ctx, co);
> +        return;
> +    }
> +
> +    if (qemu_in_coroutine()) {
> +        Coroutine *self = qemu_coroutine_self();
> +        assert(self != co);
> +        QSIMPLEQ_INSERT_TAIL(&self->co_queue_wakeup, co, co_queue_next);
> +    } else {
> +        aio_context_acquire(ctx);
> +        qemu_coroutine_enter(co);
> +        aio_context_release(ctx);

Why is it necessary to acquire AioContext here?  We're already in ctx.

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