Re: [Qemu-devel] [PATCH v2 2/2] monitor: delay monitor iothread creation

[Marc-André Lureau]

Hi

On Fri, Sep 28, 2018 at 12:02 PM Wolfgang Bumiller
<address@hidden> wrote:
>
> Commit d32749deb615 moved the call to monitor_init_globals()
> to before os_daemonize(), making it an unsuitable place to
> spawn the monitor iothread as it won't be inherited over the
> fork() in os_daemonize().
>
> We now spawn the thread the first time we instantiate a
> monitor which actually has use_io_thread == true. Therefore
> mon_iothread initialization is protected by monitor_lock.
>
> We still need to create the qmp_dispatcher_bh when not using
> iothreads, so this now still happens via
> monitor_init_globals().
>
> Signed-off-by: Wolfgang Bumiller <address@hidden>
> Fixes: d32749deb615 ("monitor: move init global earlier")
> ---
> Changes to v1:
>  - move mon_iothread declaration down to monitor_lock's declaration
>    (updating monitor_lock's coverage comment)
>  - in monitor_data_init() assert() that mon_iothread is not NULL or
>    not used instead of initializing it there, as its usage pattern is
>    so that it is a initialized once before being used, or never used
>    at all.
>  - in monitor_iothread_init(), protect mon_iothread initialization
>    with monitor_lock
>  - in monitor_init(): run monitor_ithread_init() in the `use_oob`
>    branch.
>    Note that I currently also test for mon_iothread being NULL there,
>    which we could leave this out as spawning new monitors isn't
>    something that happens a lot, but I like the idea of avoiding
>    taking a lock when not required.
>    Otherwise, I can send a v3 with this removed.
>
>  monitor.c | 49 ++++++++++++++++++++++++++++++-------------------
>  1 file changed, 30 insertions(+), 19 deletions(-)
>
> diff --git a/monitor.c b/monitor.c
> index d47e4259fd..870584a548 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -239,9 +239,6 @@ struct Monitor {
>      int mux_out;
>  };
>
> -/* Shared monitor I/O thread */
> -IOThread *mon_iothread;
> -
>  /* Bottom half to dispatch the requests received from I/O thread */
>  QEMUBH *qmp_dispatcher_bh;
>
> @@ -262,10 +259,11 @@ typedef struct QMPRequest QMPRequest;
>  /* QMP checker flags */
>  #define QMP_ACCEPT_UNKNOWNS 1
>
> -/* Protects mon_list, monitor_qapi_event_state.  */
> +/* Protects mon_list, monitor_qapi_event_state and mon_iothread. */
>  static QemuMutex monitor_lock;
>  static GHashTable *monitor_qapi_event_state;
>  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
> +IOThread *mon_iothread; /* Shared monitor I/O thread */
>
>  /* Protects mon_fdsets */
>  static QemuMutex mon_fdsets_lock;
> @@ -710,6 +708,7 @@ static void handle_hmp_command(Monitor *mon, const char 
> *cmdline);
>  static void monitor_data_init(Monitor *mon, bool skip_flush,
>                                bool use_io_thread)
>  {
> +    assert(!use_io_thread || mon_iothread);
>      memset(mon, 0, sizeof(Monitor));
>      qemu_mutex_init(&mon->mon_lock);
>      qemu_mutex_init(&mon->qmp.qmp_queue_lock);
> @@ -4453,16 +4452,11 @@ static AioContext *monitor_get_aio_context(void)
>
>  static void monitor_iothread_init(void)
>  {
> -    mon_iothread = iothread_create("mon_iothread", &error_abort);
> -
> -    /*
> -     * The dispatcher BH must run in the main loop thread, since we
> -     * have commands assuming that context.  It would be nice to get
> -     * rid of those assumptions.
> -     */
> -    qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(),
> -                                   monitor_qmp_bh_dispatcher,
> -                                   NULL);
> +    qemu_mutex_lock(&monitor_lock);
> +    if (!mon_iothread) {
> +        mon_iothread = iothread_create("mon_iothread", &error_abort);
> +    }
> +    qemu_mutex_unlock(&monitor_lock);
>  }
>
>  void monitor_init_globals(void)
> @@ -4472,7 +4466,15 @@ void monitor_init_globals(void)
>      sortcmdlist();
>      qemu_mutex_init(&monitor_lock);
>      qemu_mutex_init(&mon_fdsets_lock);
> -    monitor_iothread_init();
> +
> +    /*
> +     * The dispatcher BH must run in the main loop thread, since we
> +     * have commands assuming that context.  It would be nice to get
> +     * rid of those assumptions.
> +     */
> +    qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(),
> +                                   monitor_qmp_bh_dispatcher,
> +                                   NULL);
>  }
>
>  /* These functions just adapt the readline interface in a typesafe way.  We
> @@ -4535,6 +4537,9 @@ static void monitor_qmp_setup_handlers_bh(void *opaque)
>      monitor_list_append(mon);
>  }
>
> +/*
> + * This expects to be run in the main thread.
> + */

I read that Markus suggested that comment, but I don't really get why.

It means that callers (chardev new) should also be restricted to main thread.

>  void monitor_init(Chardev *chr, int flags)
>  {
>      Monitor *mon = g_malloc(sizeof(*mon));
> @@ -4551,6 +4556,9 @@ void monitor_init(Chardev *chr, int flags)
>              error_report("Monitor out-of-band is only supported by QMP");
>              exit(1);
>          }
> +        if (!mon_iothread) {
> +            monitor_iothread_init();
> +        }

I would call it unconditonnally, to avoid TOCTOU.

>      }
>
>      monitor_data_init(mon, false, use_oob);
> @@ -4607,7 +4615,9 @@ void monitor_cleanup(void)
>       * we need to unregister from chardev below in
>       * monitor_data_destroy(), and chardev is not thread-safe yet
>       */
> -    iothread_stop(mon_iothread);
> +    if (mon_iothread) {
> +        iothread_stop(mon_iothread);
> +    }
>

here the monitor_lock isn't taken, is there a reason worth a comment?

>      /* Flush output buffers and destroy monitors */
>      qemu_mutex_lock(&monitor_lock);
> @@ -4622,9 +4632,10 @@ void monitor_cleanup(void)
>      /* QEMUBHs needs to be deleted before destroying the I/O thread */
>      qemu_bh_delete(qmp_dispatcher_bh);
>      qmp_dispatcher_bh = NULL;
> -
> -    iothread_destroy(mon_iothread);
> -    mon_iothread = NULL;
> +    if (mon_iothread) {
> +        iothread_destroy(mon_iothread);
> +        mon_iothread = NULL;
> +    }
>  }
>
>  QemuOptsList qemu_mon_opts = {
> --
> 2.11.0
>
>
>

thanks

-- 
Marc-André Lureau