Re: [PATCH 1/2] system/main: transfer replay mutex ownership from main thread to main loop thread

["Nicholas Piggin" <npiggin@gmail.com>]

On Fri Apr 11, 2025 at 8:55 AM AEST, Pierrick Bouvier wrote:
> On MacOS, UI event loop has to be ran in the main thread of a process.
> Because of that restriction, on this platform, qemu main event loop is
> ran on another thread [1].
>
> This breaks record/replay feature, which expects thread running qemu_init
> to initialize hold this lock, breaking associated functional tests on
> MacOS.
>
> Thus, as a generalization, and similar to how BQL is handled, we release
> it after init, and reacquire the lock before entering main event loop,
> avoiding a special case if a separate thread is used.
>
> Tested on MacOS with:
> $ meson test -C build --setup thorough --print-errorlogs \
> func-x86_64-x86_64_replay func-arm-arm_replay func-aarch64-aarch64_replay
> $ ./build/qemu-system-x86_64 -nographic -icount shift=auto,rr=record,rrfile=replay.log
> $ ./build/qemu-system-x86_64 -nographic -icount shift=auto,rr=replay,rrfile=replay.log
>
> [1] https://gitlab.com/qemu-project/qemu/-/commit/f5ab12caba4f1656479c1feb5248beac1c833243
>
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/2907
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> ---
>  system/main.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/system/main.c b/system/main.c
> index ecb12fd397c..1c022067349 100644
> --- a/system/main.c
> +++ b/system/main.c
> @@ -25,6 +25,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu-main.h"
>  #include "qemu/main-loop.h"
> +#include "system/replay.h"
>  #include "system/system.h"
>  
>  #ifdef CONFIG_SDL
> @@ -44,10 +45,12 @@ static void *qemu_default_main(void *opaque)
>  {
>      int status;
>  
> +    replay_mutex_lock();
>      bql_lock();
>      status = qemu_main_loop();
>      qemu_cleanup(status);
>      bql_unlock();
> +    replay_mutex_unlock();
>  
>      exit(status);
>  }
> @@ -67,6 +70,7 @@ int main(int argc, char **argv)
>  {
>      qemu_init(argc, argv);
>      bql_unlock();
> +    replay_mutex_unlock();
>      if (qemu_main) {
>          QemuThread main_loop_thread;
>          qemu_thread_create(&main_loop_thread, "qemu_main",

Do we actually need to hold replay mutex (or even bql) over qemu_init()?
Both should get dropped before we return here. But as a simple fix, I
guess this is okay.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>