Re: [Qemu-devel] [PATCH 1/3] io: Yield rather than wait when already in coroutine

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Sep 05, 2017 at 02:11:12PM -0500, Eric Blake wrote:
> The new qio_channel_{read,write}{,v}_all functions are documented
> as yielding until data is available.  When used on a blocking
> channel, this yield is done via qio_channel_wait() which spawns
> a new coroutine under the hood (so it is the new entry point that
> yields as needed); but if we are already in a coroutine (at which

qio_channel_wait doesn't spawn any coroutine - it simply rnus a
nested event loop to wait for the channel...

> point QIO_CHANNEL_ERR_BLOCK is only possible if we are a
> non-blocking channel), we want to yield the current coroutine
> instead of spawning a new one.

...none the less, I think this is ok.

> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  io/channel.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/io/channel.c b/io/channel.c
> index 5e8c2f0a91..9e62794cab 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -105,7 +105,11 @@ int qio_channel_readv_all(QIOChannel *ioc,
>          ssize_t len;
>          len = qio_channel_readv(ioc, local_iov, nlocal_iov, errp);
>          if (len == QIO_CHANNEL_ERR_BLOCK) {
> -            qio_channel_wait(ioc, G_IO_IN);
> +            if (qemu_in_coroutine()) {
> +                qio_channel_yield(ioc, G_IO_IN);
> +            } else {
> +                qio_channel_wait(ioc, G_IO_IN);
> +            }
>              continue;
>          } else if (len < 0) {
>              goto cleanup;
> @@ -143,7 +147,11 @@ int qio_channel_writev_all(QIOChannel *ioc,
>          ssize_t len;
>          len = qio_channel_writev(ioc, local_iov, nlocal_iov, errp);
>          if (len == QIO_CHANNEL_ERR_BLOCK) {
> -            qio_channel_wait(ioc, G_IO_OUT);
> +            if (qemu_in_coroutine()) {
> +                qio_channel_yield(ioc, G_IO_OUT);
> +            } else {
> +                qio_channel_wait(ioc, G_IO_OUT);
> +            }
>              continue;
>          }
>          if (len < 0) {

Acked-by: Daniel P. Berrange <berrange@redhat.com>

Regards,
Daniel
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