Re: [Qemu-devel] [PATCH] cpus.c: Fix race condition in cpu_stop_current()

[Peter Maydell]

On Fri, 7 Dec 2018 at 16:00, Peter Maydell <address@hidden> wrote:
>
> We use cpu_stop_current() to ensure the current CPU has stopped
> from places like qemu_system_reset_request(). Unfortunately its
> current implementation has a race. It calls qemu_cpu_stop(),
> which sets cpu->stopped to true even though the CPU hasn't
> actually stopped yet. The main thread will look at the flags
> set by qemu_system_reset_request() and call pause_all_vcpus().
> pause_all_vcpus() waits for every cpu to have cpu->stopped true,
> so it can continue (and we will start the system reset operation)
> before the vcpu thread has got back to its top level loop.
>
> Instead, just set cpu->stop and call cpu_exit(). This will
> cause the vcpu to exit back to the top level loop, and there
> (as part of the wait_io_event code) it will call qemu_cpu_stop().
>
> This fixes bugs where the reset request appeared to be ignored
> or the CPU misbehaved because the reset operation started
> to change vcpu state while the vcpu thread was still using it.
>
> Signed-off-by: Peter Maydell <address@hidden>

I'm going to put this in via target-arm.next, unless anybody
would like to suggest another route.

thanks
-- PMM