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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