Re: [PATCH v6] Work around vhost-user-blk-test hang

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 14, 2021 at 04:32:23AM +0000, Raphael Norwitz wrote:
> The vhost-user-blk-test qtest has been hanging intermittently for a
> while. The root cause is not yet fully understood, but the hang is
> impacting enough users that it is important to merge a workaround for
> it.
> 
> The race which causes the hang occurs early on in vhost-user setup,
> where a vhost-user message is never received by the backend. Forcing
> QEMU to wait until the storage-daemon has had some time to initialize
> prevents the hang. Thus the existing storage-daemon pidfile option can
> be used to implement a workaround cleanly and effectively, since it
> creates a file only once the storage-daemon initialization is complete.
> 
> This change implements a workaround for the vhost-user-blk-test hang by
> making QEMU wait until the storage-daemon has written out a pidfile
> before attempting to connect and send messages over the vhost-user
> socket.
> 
> Some relevent mailing list discussions:
> 
> [1] https://lore.kernel.org/qemu-devel/CAFEAcA8kYpz9LiPNxnWJAPSjc=nv532bEdyfynaBeMeohqBp3A@mail.gmail.com/
> [2] https://lore.kernel.org/qemu-devel/YWaky%2FKVbS%2FKZjlV@stefanha-x1.localdomain/
> 
> Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>


Um. Does not seem to make things better for me:

**
ERROR:../tests/qtest/vhost-user-blk-test.c:950:start_vhost_user_blk: assertion failed (retries < PIDFILE_RETRIES): (5 < 5)
ERROR qtest-x86_64/qos-test - Bail out! ERROR:../tests/qtest/vhost-user-blk-test.c:950:start_vhost_user_blk: assertion failed (retries < PIDFILE_RETRIES): (5 < 5)

At this point I just disabled the test in meson. No need to make
everyone suffer.


> ---
>  tests/qtest/vhost-user-blk-test.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/tests/qtest/vhost-user-blk-test.c b/tests/qtest/vhost-user-blk-test.c
> index 6f108a1b62..c6626a286b 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;


Don't like the arbitrary retries counter.

Let's warn maybe, but on a busy machine we might not complete this
in time ...


> @@ -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());
> +

Ugh. Predictable paths directly in /tmp are problematic .. mktemp?

>      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 +937,27 @@ static void start_vhost_user_blk(GString *cmd_line, int vus_instances,
>          execlp("/bin/sh", "sh", "-c", storage_daemon_command->str, NULL);
>          exit(1);
>      }
> +
> +    /*
> +     * FIXME: The loop here ensures the storage-daemon has come up properly
> +     *        before allowing the test to proceed. This is a workaround for
> +     *        a race which used to cause the vhost-user-blk-test to hang. It
> +     *        should be deleted once the root cause is fully understood and
> +     *        fixed.
> +     */
> +    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