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Date: Mon, 29 Jul 2013 08:30:26 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC v2 3/5] timer: make qemu_clock_enable sync
 between disable and timer's cb
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To: Liu Ping Fan <qemulist@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Alex Bligh <alex@alex.org.uk>, Anthony Liguori <anthony@codemonkey.ws>

Il 29/07/2013 05:16, Liu Ping Fan ha scritto:
> After disabling the QemuClock, we should make sure that no QemuTimers
> are still in flight. To implement that, the caller of disabling will
> wait until the last user's exit.
> 
> Note, 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>

This is an interesting approach.

> -    if (!clock->enabled)
> -        return;
> +    atomic_inc(&clock->using);
> +    if (unlikely(!clock->enabled)) {
> +        goto exit;
> +    }

This can return directly, it doesn't need to increment and decrement
clock->using.

Paolo

>  
>      current_time = qemu_get_clock_ns(clock);
>      tlist = clock_to_timerlist(clock);
> @@ -461,6 +482,15 @@ void qemu_run_timers(QEMUClock *clock)
>          /* run the callback (the timer list can be modified) */
>          ts->cb(ts->opaque);
>      }
> +
> +exit:
> +    qemu_mutex_lock(&clock->lock);
> +    if (atomic_fetch_dec(&clock->using) == 1) {
> +        if (unlikely(!clock->enabled)) {
> +            qemu_cond_signal(&clock->wait_using);
> +        }
> +    }
> +    qemu_mutex_unlock(&clock->lock);
>  }
>  
>  int64_t qemu_get_clock_ns(QEMUClock *clock)
>