Re: [PATCH 06/22] exec/cpu: Call cpu_remove_sync() once in cpu_common_unrealize()

[Igor Mammedov]

On Mon, 18 Sep 2023 18:02:39 +0200
Philippe Mathieu-Daudé <philmd@linaro.org> wrote:

> While create_vcpu_thread() creates a vCPU thread, its counterpart
> is cpu_remove_sync(), which join and destroy the thread.
> 
> create_vcpu_thread() is called in qemu_init_vcpu(), itself called
> in cpu_common_realizefn(). Since we don't have qemu_deinit_vcpu()
> helper (we probably don't need any), simply destroy the thread in
> cpu_common_unrealizefn().
> 
> Note: only the PPC and X86 targets were calling cpu_remove_sync(),
> meaning all other targets were leaking the thread when the vCPU
> was unrealized (mostly when vCPU are hot-unplugged).
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  hw/core/cpu-common.c  | 3 +++
>  target/i386/cpu.c     | 1 -
>  target/ppc/cpu_init.c | 2 --
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c
> index a3b8de7054..e5841c59df 100644
> --- a/hw/core/cpu-common.c
> +++ b/hw/core/cpu-common.c
> @@ -221,6 +221,9 @@ static void cpu_common_unrealizefn(DeviceState *dev)
>  
>      /* NOTE: latest generic point before the cpu is fully unrealized */
>      cpu_exec_unrealizefn(cpu);
> +
> +    /* Destroy vCPU thread */
> +    cpu_remove_sync(cpu);
>  }
>  
>  static void cpu_common_initfn(Object *obj)
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index cb41d30aab..d79797d963 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -7470,7 +7470,6 @@ static void x86_cpu_unrealizefn(DeviceState *dev)
>      X86CPUClass *xcc = X86_CPU_GET_CLASS(dev);
>  
>  #ifndef CONFIG_USER_ONLY
> -    cpu_remove_sync(CPU(dev));
>      qemu_unregister_reset(x86_cpu_machine_reset_cb, dev);
>  #endif

missing  followup context:
    ...
    xcc->parent_unrealize(dev); 

Before the patch, vcpu thread is stopped and onnly then
clean up happens.

After the patch we have cleanup while vcpu thread is still running.

Even if it doesn't explode, such ordering still seems to be wrong.

> diff --git a/target/ppc/cpu_init.c b/target/ppc/cpu_init.c
> index e2c06c1f32..24d4e8fa7e 100644
> --- a/target/ppc/cpu_init.c
> +++ b/target/ppc/cpu_init.c
> @@ -6853,8 +6853,6 @@ static void ppc_cpu_unrealize(DeviceState *dev)
>  
>      pcc->parent_unrealize(dev);
>  
> -    cpu_remove_sync(CPU(cpu));

bug in current code?

> -
>      destroy_ppc_opcodes(cpu);
>  }
>