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References: <20180718154200.26777-1-dplotnikov@virtuozzo.com>
	<20180718154200.26777-8-dplotnikov@virtuozzo.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <f24de8e1-b0fd-1640-a145-7929616418ea@redhat.com>
Date: Fri, 20 Jul 2018 12:34:14 +0200
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Subject: Re: [Qemu-devel] [PATCH v1 07/17] background snapshot: introduce
 page buffer
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To: Denis Plotnikov <dplotnikov@virtuozzo.com>, dgilbert@redhat.com, quintela@redhat.com
Cc: qemu-devel@nongnu.org

On 18/07/2018 17:41, Denis Plotnikov wrote:
> +static void ram_page_buffer_decrease_used(void)
> +{
> +    qemu_event_reset(&ram_state->page_buffer.used_decreased);
> +    atomic_dec(&ram_state->page_buffer.used);
> +    qemu_event_set(&ram_state->page_buffer.used_decreased);

As Peter noted, only the qemu_event_set is needed here, while...

> +}
> +
> +static void ram_page_buffer_increase_used_wait(void)
> +{
> +    int used, *used_ptr;
> +    RAMState *rs = ram_state;
> +    used_ptr = &rs->page_buffer.used;
> +    do {
> +        used = atomic_read(used_ptr);
> +        if (rs->page_buffer.capacity > used) {
> +            if (atomic_cmpxchg(used_ptr, used, used + 1) == used) {
> +                return;
> +            } else {
> +                continue;
> +            }
> +        } else {
> +            qemu_event_wait(&rs->page_buffer.used_decreased);
> +        }
> +    } while (true);

... the right idiom for using qemu_event_wait is this:

   if test condition
       reset
       if test condition
           wait

So something like

    for(;;) {
        used = atomic_read(used_ptr);
        if (rs->page_buffer.capacity <= used) {
            qemu_event_reset(&rs->page_buffer.used_decreased);
            used = atomic_read(used_ptr);
            if (rs->page_buffer.capacity <= used) {
                qemu_event_wait(&rs->page_buffer.used_decreased);
                continue;
            }
        }

        if (atomic_cmpxchg(used_ptr, used, used + 1) == used) {
            return;
        }
    }

Note that there must be a single thread doing the above.  Otherwise, you
have to use mutex and condvar.

Paolo