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    SubjectRe: [GIT] Sparc
    On Wed, Sep 22, 2010 at 12:08:53PM -0700, Linus Torvalds wrote:
    > On Wed, Sep 22, 2010 at 11:53 AM, Al Viro <> wrote:
    > >
    > > Um, no. ?You've *already* called get_signal_to_deliver(). ?There had been
    > > no SIGSEGV in sight. ?You happily went on to set a sigframe for e.g.
    > > SIGHUP, but ran out of stack. ?At that point you get force_sigsegv()
    > > from handle_signal(). ?_NOW_ you have a pending SIGSEGV
    > Ahh. Ok. Different case from the one I thought you were worried about.
    > And yeah, I guess that one does require us to mess with the low-level
    > asm code (although I do wonder if we could not make the whole
    > do_notify_resume + reschedule code be generic C code - it's a lot of
    > duplicated subtle asm as it is).

    Worse than just that... Note that on sparc you need to deal with
    fault_in_user_windows(), which can also trigger signals. So much
    for merging it cross-architecture, even if we ignore the differences
    between the ways we pass data required for restart to do_signal().
    Note that e.g. on alpha we pass _two_ values - one for overwritten
    v0 (syscall number), another for overwritten a3 (error flag), etc.

    Worse, quite a few targets do not save all registers on syscall entry.
    Callee-saved stuff doesn't have to be in pt_regs. Except that you
    want *all* of them saved on stack when it's time to fill sigframes.
    And once you've entered a C function you can't guarantee that compiler
    hasn't already modified them; sure, they'll be restored on return, but
    that doesn't help you to get to their values. So you get switch_stack
    built on stack around calling do_notify_resume() on those. And you
    really want to avoid doing that if you've got no signals - remember,
    we hit that on exit from irqs as well, so it's one hell of a hot path.
    For processors with big flat register file it can get very ugly.

    There is a lot of ugly almost-duplication in there, no arguments about that.
    Asm glue is actually not the worst part...

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      Last update: 2010-09-22 21:57    [W:0.019 / U:28.016 seconds]
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