Re: [PATCH] qio: Inherit follow_coroutine_ctx across TLS

[Daniel P . Berrangé]

On Wed, May 15, 2024 at 09:14:06PM -0500, Eric Blake wrote:
> Since qemu 8.2, the combination of NBD + TLS + iothread crashes on an
> assertion failure:
> 
> qemu-kvm: ../io/channel.c:534: void qio_channel_restart_read(void *): 
> Assertion `qemu_get_current_aio_context() == 
> qemu_coroutine_get_aio_context(co)' failed.
> 
> It turns out that when we removed AioContext locking, we did so by
> having NBD tell its qio channels that it wanted to opt in to
> qio_channel_set_follow_coroutine_ctx(); but while we opted in on the
> main channel, we did not opt in on the TLS wrapper channel.
> qemu-iotests has coverage of NBD+iothread and NBD+TLS, but apparently
> no coverage of NBD+TLS+iothread, or we would have noticed this
> regression sooner.  (I'll add that in the next patch)
> 
> But while we could manually opt in to the TLS thread in nbd/server.c,
> it is more generic if all qio channels that wrap other channels
> inherit the follow status, in the same way that they inherit feature
> bits.
> 
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Daniel P. Berrangé <berrange@redhat.com>
> CC: qemu-stable@nongnu.org
> Fixes: https://issues.redhat.com/browse/RHEL-34786
> Fixes: 06e0f098 ("io: follow coroutine AioContext in qio_channel_yield()", 
> v8.2.0)
> Signed-off-by: Eric Blake <eblake@redhat.com>

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

> ---
> 
> Maybe we should turn ioc->follow_coroutine_ctx into a
> QIO_CHANNEL_FEATURE_* bit?

The features thus far have all been about properties of the underlying
channel, rather than properties related to the API usage pattern. So
I think it is fine to have it separate.


With regards,
Daniel
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