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References: <20190218161822.3573-1-kwolf@redhat.com>
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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <9fe39150-f308-8f2e-0c9d-6d0a31d9329d@redhat.com>
Date: Mon, 18 Feb 2019 18:22:59 +0100
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Subject: Re: [Qemu-devel] [PATCH 07/12] nbd: Increase bs->in_flight during
 AioContext switch
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To: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org
Cc: mreitz@redhat.com, eblake@redhat.com, stefanha@redhat.com, berrange@redhat.com, qemu-devel@nongnu.org

On 18/02/19 17:18, Kevin Wolf wrote:
> +            /* aio_ctx_switch is only supposed to be set if we're sitting in
> +             * the qio_channel_yield() below. */
> +            assert(!*aio_ctx_switch);
>              bdrv_dec_in_flight(bs);
>              qio_channel_yield(ioc, G_IO_IN);
> -            bdrv_inc_in_flight(bs);
> +            if (*aio_ctx_switch) {
> +                /* nbd_client_attach_aio_context() already increased in_flight
> +                 * when scheduling this coroutine for reentry */
> +                *aio_ctx_switch = false;
> +            } else {
> +                bdrv_inc_in_flight(bs);
> +            }

Hmm, my first thought would have been to do the bdrv_inc_in_flight(bs);
unconditionally here, and in nbd_connection_entry do the opposite, like

	if (s->aio_ctx_switch) {
	    s->aio_ctx_switch = false;
	    bdrv_dec_in_flight(bs);
	}

but I guess the problem is that then bdrv_drain could hang.

So my question is:

1) is there a testcase that shows the problem with this "obvious"
refactoring;

2) maybe instead of aio_co_schedul-ing client->connection_co and having
the s->aio_ctx_switch flag, you could go through a bottom half that does
the bdrv_inc_in_flight and then enters client->connection_co?

Paolo