Re: [PATCH v5] Prevent vhost-user-blk-test hang

[Stefan Hajnoczi]

On Thu, Sep 30, 2021 at 05:29:06AM +0000, Raphael Norwitz wrote:
> On Tue, Sep 28, 2021 at 10:55:00AM +0200, Stefan Hajnoczi wrote:
> > On Mon, Sep 27, 2021 at 05:17:01PM +0000, Raphael Norwitz wrote:
> > > In the vhost-user-blk-test, as of now there is nothing stoping
> > > vhost-user-blk in QEMU writing to the socket right after forking off the
> > > storage daemon before it has a chance to come up properly, leaving the
> > > test hanging forever. This intermittently hanging test has caused QEMU
> > > automation failures reported multiple times on the mailing list [1].
> > > 
> > > This change makes the storage-daemon notify the vhost-user-blk-test
> > > that it is fully initialized and ready to handle client connections by
> > > creating a pidfile on initialiation. This ensures that the storage-daemon
> > > backend won't miss vhost-user messages and thereby resolves the hang.
> > > 
> > > [1] 
> > > https://lore.kernel.org/qemu-devel/CAFEAcA8kYpz9LiPNxnWJAPSjc=nv532bEdyfynaBeMeohqBp3A@mail.gmail.com/
> > 
> 
> Hey Stefan,
> 
> > Hi Raphael,
> > I would like to understand the issue that is being worked around in the
> > patch.
> > 
> > QEMU should be okay with listen fd passing. The qemu-storage-daemon
> > documentation even contains example code for this
> > (docs/tools/qemu-storage-daemon.rst) and that may need to be updated if
> > listen fd passing is fundamentally broken.
> > 
> 
> The issue is that the "client" (in this case vhost-user-blk in QEMU) can
> proceed to use the socket before the storage-daemon has a chance to
> properly start up and monitor it. This is nothing unique to the
> storage-daemon - I've seen races like this happen happend with different
> vhost-user backends before.
> 
> Yes - I do think the docs can be improved to explicitly state that the
> storage-daemon must be allowed to properly initialize before any data is
> sent over the socket. Maybe we should even perscribe the use of the pidfile
> option?
> 
> > Can you share more details about the problem?
> > 
> 
> Did you see my analysis [1]?
> 
> [1] 
> https://lore.kernel.org/qemu-devel/20210827165253.GA14291@raphael-debian-dev/
> 
> Basically QEMU sends VHOST_USER_GET_PROTOCOL_FEATURES across the vhost
> socket and the storage daemon never receives it. Looking at the
> QEMU state we see it is stuck waiting for a vhost-user response. Meanwhile
> the storage-daemon never receives any message to begin with. AFAICT
> there is nothing stopping QEMU from running first and sending a message
> before vhost-user-blk comes up, and from testing we can see that waiting
> for the storage-daemon to come up resolves the problem completely.

The root cause has not been determined yet. QEMU should accept the
incoming connection and then read the previously-sent
VHOST_USER_GET_PROTOCOL_FEATURES message. There is no reason at the
Sockets API level why the message should get lost, so there is probably
a QEMU bug here.

> > Does "writing to the socket" mean writing vhost-user protocol messages
> > or does it mean connect(2)?
> > 
> 
> Yes - it means writing vhost-user messages. We see a message sent from
> QEMU to the backend.
> 
> Note that in qtest_socket_server() (called from create_listen_socket())
> we have already called listen() on the socket, so I would expect QEMU
> calling connect(2) to succeed and proceed to successfully send messages
> whether or not there is another listener. I even tried commenting out the
> execlp for the storage-daemon and I saw the same behavior from QEMU - it
> sends the message and hangs indefinitely.

QEMU is correct in waiting for a vhost-user reply. The question is why
qemu-storage-daemon's vhost-user-block export isn't processing the
request and replying to it?

> > Could the problem be that vhost-user-blk-test.c creates the listen fds
> > and does not close them? This means the host network stack doesn't
> > consider the socket closed after QEMU terminates and therefore the test
> > process hangs after QEMU is gone? In that case vhost-user-blk-test needs
> > to close the fds after spawning qemu-storage-daemon.
> > 
> 
> When the test hangs both QEMU and storage-daemon are still up and
> connected to the socket and waiting for messages from each other. I don't
> see how we would close the FD in this state or how it would help.

Yes, I see. In that case the theory about fds doesn't apply.

> We may want to think about implementing some kind of timeoout for initial
> vhost-user messages so that we fail instead of hang in cases like these,
> as I proposed in [1]. What do you think?

Let's hold off on workarounds until the root cause has been found.

Do you have time to debug why vu_accept() and vu_message_read() don't
see the pending VHOST_USER_GET_PROTOCOL_FEATURES message?

Thanks,
Stefan

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