Re: [PATCH v2] ui: Fix hanging up Cocoa display on macOS 10.15 (Catalina)

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 14 Oct 2019 at 15:16, <hikarupsp@gmail.com> wrote:
>
> From: Hikaru Nishida <hikarupsp@gmail.com>
>
> macOS API documentation says that before applicationDidFinishLaunching
> is called, any events will not be processed. However, some events are
> fired before it is called in macOS Catalina. This causes deadlock of
> iothread_lock in handleEvent while it will be released after the
> app_started_sem is posted.
> This patch avoids processing events before the app_started_sem is
> posted to prevent this deadlock.
>
> Buglink: https://bugs.launchpad.net/qemu/+bug/1847906
> Signed-off-by: Hikaru Nishida <hikarupsp@gmail.com>
> ---
>  ui/cocoa.m | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/ui/cocoa.m b/ui/cocoa.m
> index f12e21df6e..bccd861d16 100644
> --- a/ui/cocoa.m
> +++ b/ui/cocoa.m
> @@ -134,6 +134,7 @@
>
>  static QemuSemaphore display_init_sem;
>  static QemuSemaphore app_started_sem;
> +volatile sig_atomic_t allow_events;

Sorry, I failed to spot this on version 1 of the patch...
I think we don't need to mark this 'volatile sig_atomic_t',
but could use a simple 'bool', because both applicationDidFinishLaunching()
and handleEvent() are called from the same thread (the OSX run loop
thread).  Could you test that it still works with plain 'bool',
please?

(If we did need to handle multiple thread accesses we should
probably prefer one of the QEMU atomic primitives described
in docs/devel/atomics.txt, since 'volatile' isn't really sufficient.)

thanks
-- PMM