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Re: Cannot unwind through MIPS signal frames with ICACHE_REFILLS_WORKAROUND_WAR
On Nov 13, 2007 3:00 PM, Ralf Baechle <ralf@linux-mips.org> wrote:
>
> On Tue, Nov 13, 2007 at 02:14:58PM +0100, Franck Bui-Huu wrote:
>
> > > > David Daney writes:
> > > > > With the current kernel (2.6.23.1) in my R5000 based O2 it seems
> > > > > impossible for GCC's exception unwinding machinery to unwind through
> > > > > signal frames. The cause of the problems is the
> > > > > ICACHE_REFILLS_WORKAROUND_WAR which puts the sigcontext at an almost
> > > > > impossible to determine offset from the signal return trampoline. The
> > > > > unwinder depends on being able to find the sigcontext given a known
> > > > > location of the trampoline.
> > > > >
> > > > > It seems there are a couple of possible solutions:
> > > > >
> > > > > 1) The comments in war.h indicate the problem only exists in R7000
> > > > > and E9000 processors. We could turn off the workaround if the
> > > > > kernel is configured for R5000. That would help me, but not those
> > > > > with the effected systems.
> > > > >
> > > > > 2) In the non-workaround case, the siginfo immediately follows the
> > > > > trampoline and the first member is the signal number. For the
> > > > > workaround case the first word following the trampoline is zero.
> > > > > We could replace this with the offset to the sigcontext which is
> > > > > always a small negative value. The unwinder could then distinguish
> > > > > the two cases (signal numbers are positive and the offset
> > > > > negative). If we did this, the change would have to be coordinated
> > > > > with GCC's unwinder (in libgcc_s.so.1).
> > > > >
> > > > > Thoughts?
> > > >
> > > > The best solution is to put the unwinder info in the kernel. Does
> > > > MIPS use a vDSO ?
> > >
> > > No though we should.
> > >
> > > Another reason is to get rid of the classic trampoline the kernel installs
> > > on the stack. On some multiprocessor systems it requires a cacheflush
> > > operation to be performed on all processors which is expensive. Having
> > > the trampoline in a vDSO would solve that.
> > >
> >
> > And the stack wouldn't need to have exec permission anymore.
>
> Oh?
>
> extern void frob(void (*)(void));
>
> int foo(void)
> {
> int x;
>
> void bar(void)
> {
> x++;
> }
>
> frob(&bar);
> print("x is %d\n", x);
> }
>
> Compile and enjoy.
>
Sorry Ralf, I missed your point.
--
Franck