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=nv532bedyfynabemeohqb...@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. > 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. > 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. 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? > Stefan > > > > > Signed-off-by: Raphael Norwitz <raphael.norw...@nutanix.com> > > Reviewed-by: Eric Blake <ebl...@redhat.com> > > --- > > tests/qtest/vhost-user-blk-test.c | 26 +++++++++++++++++++++++++- > > 1 file changed, 25 insertions(+), 1 deletion(-) > > > > diff --git a/tests/qtest/vhost-user-blk-test.c > > b/tests/qtest/vhost-user-blk-test.c > > index 6f108a1b62..5fed262da1 100644 > > --- a/tests/qtest/vhost-user-blk-test.c > > +++ b/tests/qtest/vhost-user-blk-test.c > > @@ -24,6 +24,7 @@ > > #define TEST_IMAGE_SIZE (64 * 1024 * 1024) > > #define QVIRTIO_BLK_TIMEOUT_US (30 * 1000 * 1000) > > #define PCI_SLOT_HP 0x06 > > +#define PIDFILE_RETRIES 5 > > > > typedef struct { > > pid_t pid; > > @@ -885,7 +886,8 @@ static void start_vhost_user_blk(GString *cmd_line, int > > vus_instances, > > int num_queues) > > { > > const char *vhost_user_blk_bin = qtest_qemu_storage_daemon_binary(); > > - int i; > > + int i, retries; > > + char *daemon_pidfile_path; > > gchar *img_path; > > GString *storage_daemon_command = g_string_new(NULL); > > QemuStorageDaemonState *qsd; > > @@ -898,6 +900,8 @@ static void start_vhost_user_blk(GString *cmd_line, int > > vus_instances, > > " -object memory-backend-memfd,id=mem,size=256M,share=on " > > " -M memory-backend=mem -m 256M "); > > > > + daemon_pidfile_path = g_strdup_printf("/tmp/daemon-%d", getpid()); > > + > > for (i = 0; i < vus_instances; i++) { > > int fd; > > char *sock_path = create_listen_socket(&fd); > > @@ -914,6 +918,9 @@ static void start_vhost_user_blk(GString *cmd_line, int > > vus_instances, > > i + 1, sock_path); > > } > > > > + g_string_append_printf(storage_daemon_command, "--pidfile %s ", > > + daemon_pidfile_path); > > + > > g_test_message("starting vhost-user backend: %s", > > storage_daemon_command->str); > > pid_t pid = fork(); > > @@ -930,7 +937,24 @@ static void start_vhost_user_blk(GString *cmd_line, > > int vus_instances, > > execlp("/bin/sh", "sh", "-c", storage_daemon_command->str, NULL); > > exit(1); > > } > > + > > + /* > > + * Ensure the storage-daemon has come up properly before allowing the > > + * test to proceed. > > + */ > > + retries = 0; > > + while (access(daemon_pidfile_path, F_OK) != 0) { > > + g_assert_cmpint(retries, <, PIDFILE_RETRIES); > > + > > + retries++; > > + g_usleep(1000); > > + } > > + > > g_string_free(storage_daemon_command, true); > > + if (access(daemon_pidfile_path, F_OK) == 0) { > > + unlink(daemon_pidfile_path); > > + } > > + g_free(daemon_pidfile_path); > > > > qsd = g_new(QemuStorageDaemonState, 1); > > qsd->pid = pid; > > -- > > 2.20.1 > >
