Re: [Qemu-devel] [PATCH v6 2/2] vl: fix use of --daemonize with --preconfig

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jun 07, 2018 at 02:00:09PM +0200, Igor Mammedov wrote:
> When using --daemonize, the initial lead process will fork a child and
> then wait to be notified that setup is complete via a pipe, before it
> exits.  When using --preconfig there is an extra call to main_loop()
> before the notification is done from os_setup_post(). Thus the parent
> process won't exit until the mgmt application connects to the monitor
> and tells QEMU to leave the RUN_STATE_PRECONFIG. The mgmt application
> won't connect to the monitor until daemonizing has completed though.
> 
> This is a chicken and egg problem, leading to deadlock at startup.
> 
> The only viable way to fix this is to call os_setup_post() before
> the early main_loop() call when --preconfig is used. This has the
> downside that any errors from this point onwards won't be handled
> well by the mgmt application, because it will think QEMU has started
> successfully, so not be expecting an abrupt exit. Moving as much user
> input validation as possible to before the main_loop() call might help,
> but mgmt application should stop assuming that QEMU has started
> successfuly and use other means to collect errors from QEMU (logfile).
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v5:
>   * use original Daniel's patch [1], but addapt it to apply on top of
>     "[PATCH v3 1/2] cli: Don't run early event loop if no  --preconfig was specified"
>     with extra comment and massage commit message a little bit.
> v6:
>   * hide os_setup_post_done flag inside of os_setup_post() as it was in v4
> 
> CC: berrange@redhat.com
> CC: mreitz@redhat.com
> CC: pbonzini@redhat.com
> CC: ehabkost@redhat.com
> CC: ldoktor@redhat.com
> CC: eblake@redhat.com
> ---
>  os-posix.c | 6 ++++++
>  vl.c       | 6 ++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/os-posix.c b/os-posix.c
> index 9ce6f74..0246195 100644
> --- a/os-posix.c
> +++ b/os-posix.c
> @@ -309,8 +309,14 @@ void os_daemonize(void)
>  
>  void os_setup_post(void)
>  {
> +    static bool os_setup_post_done;
>      int fd = 0;
>  
> +    if (os_setup_post_done) {
> +        return;
> +    }
> +    os_setup_post_done = true;
> +
>      if (daemonize) {
>          if (chdir("/")) {
>              error_report("not able to chdir to /: %s", strerror(errno));
> diff --git a/vl.c b/vl.c
> index fa44138..457ff2a 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -4578,6 +4578,12 @@ int main(int argc, char **argv, char **envp)
>      parse_numa_opts(current_machine);
>  
>      /* do monitor/qmp handling at preconfig state if requested */
> +    if (!preconfig_exit_requested && is_daemonized()) {
> +        /* signal parent QEMU to exit, libvirt treats it as a sign
> +         * that monitor socket is ready to accept connections
> +         */
> +        os_setup_post();
> +    }

I was looking at the daemonize logic, and noticed it we have a
huge amount of code between this line and the next
os_setup_post() call that could either:

* call exit() and/or error_report(); or
* be unable to finish machine initialization because of
  chdir("/"), change_root(), or change_process_uid().

Doesn't this make -preconfig and -daemonize fundamentally
incompatible?


>      main_loop();
>  
>      /* from here on runstate is RUN_STATE_PRELAUNCH */
> -- 
> 2.7.4
> 
> 

-- 
Eduardo