Re: [Qemu-devel] linux-user crashing in multi-threaded programs

[Peter Maydell]

On 21 November 2011 02:05, Alexander Graf <address@hidden> wrote:
> In an adventure to find out why yast2-core fails in its testsuite during 
> build on ARM for us, I dove into the code for a bit and debugged it down to 
> our old friend: multi-threaded TB invalidation.

https://bugs.launchpad.net/qemu/+bug/668799

> I have a nice test case at
>
>  http://csgraf.de/tmp/yast2-core.tbz2
>
> which you can easily use to reproduce the race.

There's also a test case attached to 668799.

> Does anyone have pending patches, approaches or ideas on how to
> fix this? It seems to be the last big road block towards getting
> linux-user actually workable for us.

I think I sent you my work-in-progress patch, didn't I?
The basic idea is that we throw away this code which tries
to unlink TB chains, and instead we just have a simple flag
which we check at the start of each TB (cost: a few insns
and a load from something that will be in L1), and a
thread wanting to cause a cpu exit simply sets the flag.

There are a couple of things in the patch that needed cleaning
up but the main reason I didn't get round to doing anything with
it was that I wanted to benchmark it for speed and I don't
have a nice benchmarking setup... (my very rough tests
suggested ~3% slowdown which personally I think is an
acceptable tradeoff for reliability.)

-- PMM