[PATCH][BZ #16159] Do not backtrace recursively.

[Ondřej Bílka]

On Mon, Nov 18, 2013 at 11:03:53AM +0000, Thorsten Glaser wrote:
> Ondřej Bílka <neleai <at> seznam.cz> writes:
> 
> > --- a/ports/sysdeps/m68k/backtrace.c
> > +++ b/ports/sysdeps/m68k/backtrace.c
> 
> > +  static __thread int recursed = 0;
> 
> m68k has a huge penalty for TLS (at least one syscall).
> 
Ok, so we need more involved code. A tls access is only needed not to
run __libc_once twice initialization routine contains dlopen/lock which is
even slower. For that we need to define reentrant version of
libc_once and handle its failures.

A second question would be why you have slow TLS? As m68k are mostly
single core you could add a alternative TLS access without syscall
overhead.

You need to add in kernel a syscall where you specify a static TCB address
and kernel will save/restore it on context switches.

To support possible future multithread processors you could for example
check if a TCB pointer is null and proceed with syscall.