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Re: [PATCH 1/4] vhost-user: Introduce nested event loop in vhost_user_re
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH 1/4] vhost-user: Introduce nested event loop in vhost_user_read() |
Date: |
Tue, 9 Mar 2021 15:02:48 +0000 |
On Mon, Mar 08, 2021 at 01:31:38PM +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.
>
> As a preliminary step to address this, split vhost_user_read() into a
> nested even loop and a one-shot callback that does the actual reading.
In case you respin:
s/even/event/
> +static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
> +{
> + struct vhost_user *u = dev->opaque;
> + CharBackend *chr = u->user->chr;
> + GMainContext *prev_ctxt = chr->chr->gcontext;
> + GMainContext *ctxt = g_main_context_new();
> + GMainLoop *loop = g_main_loop_new(ctxt, FALSE);
> + struct vhost_user_read_cb_data data = {
> + .dev = dev,
> + .loop = loop,
> + .msg = msg,
> + .ret = 0
> + };
> +
> + /* Switch context and add a new watch to monitor chardev activity */
> + qemu_chr_be_update_read_handlers(chr->chr, ctxt);
> + qemu_chr_fe_add_watch(chr, G_IO_IN | G_IO_HUP, vhost_user_read_cb,
> &data);
This comment could be expanded to explain why the nested event loop is
necessary. The goal is to monitor the slave_fd while waiting for chr
I/O so we'll need an event loop. prev_ctxt cannot be run nested since
its fd handlers may not be prepared (e.g. re-entrancy).
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[PATCH 3/4] vhost-user: Monitor slave channel in vhost_user_read(), Greg Kurz, 2021/03/08