Re: Thread safety of coroutine-sigaltstack

[Laszlo Ersek <lersek@redhat.com>]

On 01/22/21 11:14, Peter Maydell wrote:
> 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.

Yes, my grep found those occurrences of SIGUSR2 of course.

> OTOH do we even use the coroutine code in user-emulation mode?

I assumed not. I assumed coroutines were only used by the block system,
and user mode emulation is about running userspace Linux programs --
virtual disks are not emulated for those.

> 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.

Huh.

> Which is odd and probably worth fixing, but does mean we
> know that we aren't actually using coroutines in user-mode.

Thanks for checking!
Laszlo

> (Also user-mode really means Linux or BSD and I think both of
> those have working ucontext.)
>
> thanks
> -- PMM
>