Re: [Qemu-devel] [RFC v6 14/27] monitor: let suspend_cnt be thread safe

[Eric Blake]

On 12/19/2017 02:45 AM, Peter Xu wrote:
> Monitor code now can be run in more than one thread.  Let it be thread
> safe when accessing suspend_cnt counter.
> 
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  monitor.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/monitor.c b/monitor.c
> index cf1e5d881c..844508d134 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -3862,7 +3862,7 @@ static int monitor_can_read(void *opaque)
>  {
>      Monitor *mon = opaque;
>  
> -    return (mon->suspend_cnt == 0) ? 1 : 0;
> +    return (atomic_mb_read(&mon->suspend_cnt) == 0) ? 1 : 0;

Worth a comment at the declaration of suspend_cnt that it must be
accessed atomically?

I find anything that does '(boolexpr) ? 1 : 0' to be a pointless waste
of typing.  It is semantically equivalent and shorter to write either of:

return atomic_mb_read(&mon->suspend_cnt) == 0;
return !atomic_mb_read(&mon->suspend_cnt);

At any rate, I checked that all uses of suspend_cnt were converted over
to atomic API, so this looks correct, whether or not you add my tweaks.

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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