Re: [Devel] Re: [PATCH 08/10] Introduce functions to restart aprocess

From: Louis Rilling
Date: Wed Oct 22 2008 - 06:44:59 EST

On Wed, Oct 22, 2008 at 12:06:19PM +0200, Greg Kurz wrote:
> On Wed, 2008-10-22 at 11:25 +0200, Louis Rilling wrote:
> > Do you checkpoint uninterruptible syscalls as well? If only interruptible
> > syscalls are checkpointed, I'd say that either this syscall uses ERESTARTSYS or
> > ERESTART_RESTARTBLOCK, and then signal handling code already does the trick, or
> > this syscall does not restart itself when interrupted, and well, this is life,
> > userspace just sees -EINTR, which is allowed by the syscall spec.
> > Actually this is how we checkpoint/migrate tasks in interruptible syscalls in
> > Kerrighed and this works.
> >
> > Louis
> >
> I don't know Kerrighed internals but I understand you perform checkpoint
> with a signal handler. Right ?

Right. This is an kernel-internal-only signal, so all signals remain available
for userspace.

> This approach has a huge benefit: the
> signal handling code do all the arch dependant stuff to save registers
> in user memory.

Hm, I'm not sure to understand what you mean here. We just rely on arch code
that jumps to signal handling to correctly setup struct pt_regs, which is then
passed to the checkpoint code. So yes, userspace registers are mostly saved by
existing arch code. But in x86-64 for instance, segment registers still need to
be saved by the checkpoint code (a bit like copy_thread() does), and I don't
know arch-independent functions doing this.


Dr Louis Rilling Kerlabs
Skype: louis.rilling Batiment Germanium
Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes 35700 Rennes

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