Re: [Qemu-devel] [Bug 902148] Re: qemu-img V1.0 hangs on creating Image (0.15.1 runs)

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, Jan 14, 2012 at 3:40 PM, Zhi Yong Wu <zwu.kernel@gmail.com> wrote:
> On Mon, Jan 9, 2012 at 9:00 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>> On Mon, Jan 9, 2012 at 11:25 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>>> Am 20.12.2011 17:49, schrieb Stefan Hajnoczi:
>>>> On Tue, Dec 20, 2011 at 3:25 PM, Michael Niehren
>>>> <902148@bugs.launchpad.net> wrote:
>>>>> here we are. Attached the tgz. I am using no spezial distribution, it's a self compiled LFS with
>>>>> gcc V4.5.1
>>>>>
>>>>> Is there a different compiler-call if i use --enable-debug, which then
>>>>> works ?
>>>>
>>>> Richard Sandiford looked at your gcc -fdump-tree-all-details output
>>>> and suggests that this bug has been fixed in gcc 4.5.3:
>>>>
>>>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45967
>>>>
>>>> Using the most recent gcc should fix the issue you are seeing.
>>>
>>> Can we add some workaround? Not sure what will work, maybe a simple
>>> compiler barrier?
>>
>> Sure though it seems like a very rare case - OP was running Linux From
>> Scratch and hence got the broken gcc.  But if someone does a small
>> workaround and tests it then that would be nice.
> How to do this workaround in qemu since it is one gcc bug?

The problem is that that compiler keeps values in registers across a
point where C semantics require that they be reloaded.

There are several ways to force a compiler to reload values including
a barrier (which Kevin suggested) or the volatile keyword.  One of
these techniques can probably be used as a workaround, but it would be
necessary to check gcc 4.5.1 output to make sure it's effective.

I think it's not worth doing unless we think more users will be
affected.  Unless a distro ships the broken compiler version or it's
the latest gcc release that people would build from source, I bet the
number of users is very small.

Stefan