Single stepping through atomic ops

[Rafal Jaworowski]

Hi Marcel,
I'm writing to you primarily as the gdb maintainer :-) There's a  
problem with current gdb in base when single stepping through atomic  
sequences on PowerPC (and presumably other archs with the load +  
conditional store approach for atomic ops). The effect is a hang  
because we endlessly loop due to the [always] lost reservation at the  
time of a closing stwcx.

This is a known problem with gdb and it's fixed with newer versions,  
but the patches are not applicable to our gdb due to some  
infrastructure changes. The quick fix for 6.1.1 is here:
http://people.freebsd.org/~raj/patches/powerpc/gdb-ppc-single-step.diff

It's mainly a transplant of two fixes from the gdb repository (1.275,  
1.276, deal_with_atomic_sequence) adjusted to our older gdb code, but  
it's ugly #ifdef'ed __ppc__. In order to do it cleanly we would need  
to change the signature of the SOFTWARE_SINGLE_STEP method so it  
returns a value (this is how newer gdb works), but it would affect gdb  
for all architectures. Do you see any objections for growing a return  
value here?

Another direction would be importing a newer gdb code base, and be  
able to apply the fixes directly and cleanly (if at all required), but  
am not sure if there are any plans to upgrade gdb in base? Let me know  
your comments.

Rafal