Re: Thread safety of coroutine-sigaltstack

[Peter Maydell]

On Sat, 23 Jan 2021 at 00:06, Laszlo Ersek <lersek@redhat.com> wrote:
> On 01/22/21 11:14, Peter Maydell wrote:
> > 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,
>
> I did that, first running
>
>  ./configure \
>     --enable-debug \
>     --target-list==x86_64-softmmu,x86_64-linux-user \
>     --with-coroutine=sigaltstack
>
> Then I checked the "qemu-system-x86_64" and "qemu-x86_64" binaries with
> "nm". Only the former contains "coroutine_init":
>
> 00000000009725e4 t coroutine_init
>
> So I believe the coroutine object file(s) are not even linked into the
> user-mode emulators. (coroutine_init() is a constructor function, so I
> think it would be preserved otherwise, even if it had no explicit caller.)

I think the linker will only pull in a .o file from a static
library (for us, libqemuutil.a) if there's an explicit reference
to a real function that it needs; it won't pull it in merely because
it has a constructor function in it. I can't offhand find an official
docs reference for this, but here's a stack overflow question:
https://stackoverflow.com/questions/6589772/gcc-functions-with-constructor-attribute-are-not-being-linked

You can also touch the coroutine source file and watch the
build system rebuild the usermode binary. If you do a complete build,
then touch util/coroutine-ucontext.c, then do 'make -C builddir qemu-arm'
(or whatever usermode emulator you've configured) then:
 * we build coroutine-ucontext.c into
libqemuutil.a.p/util_coroutine-ucontext.c.o
 * we blow away libqemuutil.a and then run 'ar' to put a lot of .o
   files into it, including libqemuutil.a.p/util_coroutine-ucontext.c.o
 * we do some things with scripts/undefsym.py that aren't important here
 * we re-link qemu-arm with all its directly used objects and also with
   libqemuutil.a

So right now we aren't actually using the coroutine functions, but
if you did put in a call to one in code used by the usermode
emulator then that code would only fail to compile if you
were building the usermode emulators without either the tools
or the system emulators.

thanks
-- PMM