Re: [Qemu-devel] qemu-arm-static, multiprocess, atomic

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 25/05/16 15:34, Jeff Epler wrote:
> Hi.  Before I take the time to prepare a detailed bug report, I wanted to
> ask whether I'm doing something that is expected to work, or expected not
> to work.
>
> I am a developer of the LinuxCNC, Free and Open Source software for control
> of milling machines, lathes, and so forth.
>
> Our software uses an IPC model where multiple processes share a
> memory-mapped region which contains data as well as mutexes to manipulate
> the data (using gcc intrinsics like
> __sync_fetch_and_or).
>
> On real multiprocessor ARM systems, our testsuite passes, but when running
> in a debian schroot using qemu-arm-static it regularly deadlocks,
> apparently with two processes both trying to take the same mutex in a
> shared memory region.
>
> From what I have been able to tell, the emulation of atomics in qemu is
> based on a single-process, multi-thread model (based on start_exclusive()
> in linux-user/main.c,
> http://git.qemu.org/?p=qemu.git;a=blob;f=linux-user/main.c;h=95ed11d85c1#l160)
> so right now I think that our code is expected not to work on qemu-user.
> If that's the case, for our purposes I will simply document that while our
> software can be built in a qemu-user environment, it cannot be used and its
> testsuite experiences spurious failures.
>
> Thanks for your time,
> Jeff

Hi Jeff,

Looks like you are right that ARM exclusive access implementation in
QEMU implies a single-process, multi-thread model and it's not supposed
to work for your use-case. I'm afraid even our recent efforts in
multi-threaded TCG won't change the situation. The problem is that it
would require to translate somehow ARM's exclusive access monitor to x86
model.

Kind regards,
Sergey