Re: Thread safety of coroutine-sigaltstack

[Peter Maydell]

On Fri, 22 Jan 2021 at 08:50, Max Reitz <mreitz@redhat.com> wrote:
>
> On 20.01.21 18:25, Laszlo Ersek wrote:
>
> [...]
>
> > A simple grep for SIGUSR2 seems to indicate that SIGUSR2 is not used by
> > system emulation for anything else, in practice. Is it possible to
> > dedicate SIGUSR2 explicitly to coroutine-sigaltstack, and set up the
> > action beforehand, from some init function that executes on a "central"
> > thread, before qemu_coroutine_new() is ever called?
>
> I wrote a patch to that effect, but just before sending I wondered
> whether SIGUSR2 cannot be registered by the “guest” in user-mode
> emulation, and whether that would then break coroutines from there on.
>
> (I have no experience dealing with user-mode emulation, but it does look
> like the guest can just register handlers for any signal but SIGSEGV and
> SIGBUS.)

Yes, SIGUSR2 is for the guest in user-emulation mode. OTOH do we
even use the coroutine code in user-emulation mode? Looking at
the meson.build files, we only add the coroutine_*.c to util_ss
if 'have_block', and we set have_block = have_system or have_tools.
I think (but have not checked) that that means we will build and
link the object file into the user-mode binaries if you happen
to build them in the same run as system-mode binaries, but won't
build them in if you built the user-mode binaries as a separate
build. Which is odd and probably worth fixing, but does mean we
know that we aren't actually using coroutines in user-mode.
(Also user-mode really means Linux or BSD and I think both of
those have working ucontext.)

thanks
-- PMM