Re: [PATCH v2 5/7] vhost-user: Introduce nested event loop in vhost_user_read()

[Stefan Hajnoczi]

On Fri, Mar 12, 2021 at 10:22:10AM +0100, Greg Kurz wrote:
> A deadlock condition potentially exists if a vhost-user process needs
> to request something to QEMU on the slave channel while processing a
> vhost-user message.
> 
> This doesn't seem to affect any vhost-user implementation so far, but
> this is currently biting the upcoming enablement of DAX with virtio-fs.
> The issue is being observed when the guest does an emergency reboot while
> a mapping still exits in the DAX window, which is very easy to get with
> a busy enough workload (e.g. as simulated by blogbench [1]) :
> 
> - QEMU sends VHOST_USER_GET_VRING_BASE to virtiofsd.
> 
> - In order to complete the request, virtiofsd then asks QEMU to remove
>   the mapping on the slave channel.
> 
> All these dialogs are synchronous, hence the deadlock.
> 
> As pointed out by Stefan Hajnoczi:
> 
> When QEMU's vhost-user master implementation sends a vhost-user protocol
> message, vhost_user_read() does a "blocking" read during which slave_fd
> is not monitored by QEMU.
> 
> The natural solution for this issue is an event loop. The main event
> loop cannot be nested though since we have no guarantees that its
> fd handlers are prepared for re-entrancy.
> 
> Introduce a new event loop that only monitors the chardev I/O for now
> in vhost_user_read() and push the actual reading to a one-shot handler.
> A subsequent patch will teach the loop to monitor and process messages
> from the slave channel as well.
> 
> [1] https://github.com/jedisct1/Blogbench
> 
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> v2: - Document why a nested loop is needed in vhost_user_read() (Stefan)
> ---
>  hw/virtio/vhost-user.c | 65 ++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 60 insertions(+), 5 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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