Re: [Qemu-devel] qemu_futex_wait() lockups in ARM64: 2 possible issues

[Jan Glauber]

On Wed, Oct 02, 2019 at 11:45:19AM +0200, Paolo Bonzini wrote:
> On 02/10/19 11:23, Jan Glauber wrote:
> > I've tried to verify me theory with this patch and didn't run into the
> > issue for ~500 iterations (usually I would trigger the issue ~20 
> > iterations).
> 
> Awesome!  That would be a compiler bug though, as atomic_add and atomic_sub
> are defined as sequentially consistent:
> 
> #define atomic_add(ptr, n) ((void) __atomic_fetch_add(ptr, n, 
> __ATOMIC_SEQ_CST))
> #define atomic_sub(ptr, n) ((void) __atomic_fetch_sub(ptr, n, 
> __ATOMIC_SEQ_CST))

Compiler bug sounds kind of unlikely...

> What compiler are you using and what distro?  Can you compile util/aio-posix.c
> with "-fdump-rtl-all -fdump-tree-all", zip the boatload of debugging files and
> send them my way?

This is on Ubuntu 18.04.3,
gcc version 7.4.0 (Ubuntu/Linaro 7.4.0-1ubuntu1~18.04.1)

I've uploaded the debug files to:
https://bugs.launchpad.net/qemu/+bug/1805256/+attachment/5293619/+files/aio-posix.tar.xz

Thanks,
Jan

> Thanks,
> 
> Paolo