Re: [Qemu-devel] [PATCH v4 4/4] timer: make qemu_clock_enable sync between disable and timer's cb

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-09-22 10:11, Liu Ping Fan wrote:
> After disabling the QemuClock, we should make sure that no QemuTimers
> are still in flight. To implement that with light overhead, we resort
> to QemuEvent. The caller of disabling will wait on QemuEvent of each
> timerlist.
> 
> Note, qemu_clock_enable(foo,false) can _not_ be called from timer's cb.
> And the callers of qemu_clock_enable() should be sync by themselves,
> not protected by this patch.
> 
> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> ---
>  include/qemu/timer.h |  4 ++++
>  qemu-timer.c         | 20 +++++++++++++++++++-
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/include/qemu/timer.h b/include/qemu/timer.h
> index e4934dd..b26909a 100644
> --- a/include/qemu/timer.h
> +++ b/include/qemu/timer.h
> @@ -185,6 +185,10 @@ void qemu_clock_notify(QEMUClockType type);
>   * @enabled: true to enable, false to disable
>   *
>   * Enable or disable a clock
> + * Disabling the clock will wait for related timerlists to stop
> + * executing qemu_run_timers.  Thus, this functions should not
> + * be used from the callback of a timer that is based on @clock.
> + * Doing so would cause a deadlock.
>   */
>  void qemu_clock_enable(QEMUClockType type, bool enabled);
>  
> diff --git a/qemu-timer.c b/qemu-timer.c
> index 95ff47f..c500a76 100644
> --- a/qemu-timer.c
> +++ b/qemu-timer.c
> @@ -45,6 +45,7 @@
>  /* timers */
>  
>  typedef struct QEMUClock {
> +     /* We rely on BQL to protect the timerlists */
>      QLIST_HEAD(, QEMUTimerList) timerlists;
>  
>      NotifierList reset_notifiers;
> @@ -70,6 +71,8 @@ struct QEMUTimerList {
>      QLIST_ENTRY(QEMUTimerList) list;
>      QEMUTimerListNotifyCB *notify_cb;
>      void *notify_opaque;
> +    /* light weight method to mark the end of timerlist's running */
> +    QemuEvent ev;

What about "timers_done_ev"?

>  };
>  
>  /**
> @@ -98,6 +101,7 @@ QEMUTimerList *timerlist_new(QEMUClockType type,
>      QEMUClock *clock = qemu_clock_ptr(type);
>  
>      timer_list = g_malloc0(sizeof(QEMUTimerList));
> +    qemu_event_init(&timer_list->ev, false);
>      timer_list->clock = clock;
>      timer_list->notify_cb = cb;
>      timer_list->notify_opaque = opaque;
> @@ -140,13 +144,24 @@ void qemu_clock_notify(QEMUClockType type)
>      }
>  }
>  
> +/* Disabling the clock will wait for related timerlists to stop
> + * executing qemu_run_timers.  Thus, this functions should not
> + * be used from the callback of a timer that is based on @clock.
> + * Doing so would cause a deadlock.
> + */
>  void qemu_clock_enable(QEMUClockType type, bool enabled)
>  {
>      QEMUClock *clock = qemu_clock_ptr(type);
> +    QEMUTimerList *tl;
>      bool old = clock->enabled;
>      clock->enabled = enabled;
>      if (enabled && !old) {
>          qemu_clock_notify(type);
> +    } else if (!enabled && old) {
> +        /* We rely on BQL to protect the timerlists */

So the caller of qemu_clock_enable has to hold the BQL? Then please add
that to the function description above instead.

Jan

> +        QLIST_FOREACH(tl, &clock->timerlists, list) {
> +            qemu_event_wait(&tl->ev);
> +        }
>      }
>  }
>  
> @@ -373,8 +388,10 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
>      QEMUTimer *ts;
>      int64_t current_time;
>      bool progress = false;
> -   
> +
> +    qemu_event_reset(&timer_list->ev);
>      if (!timer_list->clock->enabled) {
> +        qemu_event_set(&timer_list->ev);
>          return progress;
>      }
>  
> @@ -392,6 +409,7 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
>          ts->cb(ts->opaque);
>          progress = true;
>      }
> +    qemu_event_set(&timer_list->ev);
>      return progress;
>  }
>  
> 


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