[ros-users] Ros on Arm Hanging

[Daniel Stonier]

This seems to be a bit of problem code with regards to the alignment issues:

roscpp/src/libros/service_server_link.cpp

*****************************************************************************************

void ServiceServerLink::onResponseOkAndLength(const ConnectionPtr& conn,
const boost::shared_array<uint8_t>& buffer, uint32_t size, bool success)
{
ROS_ASSERT(conn == connection_);
ROS_ASSERT(size == 5);

if (!success)
return;

uint8_t ok = buffer[0];
uint32_t len = *((uint32_t*)(buffer.get() + 1));

*****************************************************************************************

That last line is where the signal is caught. I think the problem is, the
arm core reads the uint32 values only at alignmed 4 byte boundaries - but
that +1 generally shifts it off the 4 byte boundary and causes the signal.

Do intel machines even have an alignment trap like this arm core does? I
noticed my machine does not have /proc/cpu/alignment.

Sorry, don't have a potential fix for it as I'm not familiar with this
section of code yet.

More to come,
Regards,
Daniel.


On 12 July 2010 19:04, Daniel Stonier <d.stonier at gmail.com> wrote:

>
>
> On 12 July 2010 17:24, René Wagner <rene.wagner at dfki.de> wrote:
>
>> On Fri, 2010-07-09 at 10:45 +0900, Daniel Stonier wrote:
>> > - Patched (using the memcpy's rather than the reinterprets in the
>> > SERIALIZER macro).
>> > - In signal+warn mode (echo 5 > /proc/cpu/alignment)
>> >   - (float64&int32) client gives a bus error and kernel outputs an
>> > alignment trap message, but servers are fine
>> > - In default mode (echo 0 > /proc/cpu/alignment)
>> >   - (float64&int32) client and server function without hanging like
>> before
>> >
>> > So...its working, but is this the right fix?
>>
>> You probably want to check where the unaligned memory access is
>> happening. Can you run your float_client from within gdb? This should
>> allow you to get a backtrace once the kernel has sent the signal (bus
>> error) from the alignment trap.
>>
>> Cheers,
>>
>> Rene
>>
>> Getting some more meaningful backtraces now - catch bus errors for both
> the float64's and the int32's in some cases, some with, some without that
> memcopy patch above. I'll put together some results when I get back to work
> in the morning.
>
>
>
>
>> -
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>
>
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