Re: Preemption in do_cpu (Re: [PATCH]Preemption patch for 2.6)

[Manish Lachwani <mlachwani@mvista.com>]

Jun Sun wrote:

> On Mon, Apr 18, 2005 at 01:32:46PM +0400, Pavel Kiryukhin wrote:
>  
>
> > Hi,
> > the preempt_disable/preempt_enable sequence in do_cpu() [traps.c]
> > exists quite long (patch submitted in Oct. 2004), so it should be nothing
> > wrong there.
> >
> > Can somebody please comment why use of preempt_disable/enable in do_cpu
> > will not result in "scheduling while atomic" for fpu-less cpu (with enabled
> > preemption).
> >
> > The sequence looks like
> >
> > do_cpu()
> > | preempt_disable()
> > | fpu_emulator_cop1Handler()
> > | | cond_reshed()
> > | | | schedule()  <------ scheduling while atomic
> >
> >
> > The proposed patch was tested for Sibyte, but it has fpu (AFAIK) and has no
> > fpu_emulator_cop1Handler called.
> >
> >    
> fpu_emulator maintains global variables and in general is dangerous
> to be preempted in the middle of processing.
>
> The quick fix for this problem is probably to move preemption disabling/
> enabling inside fpu_emulator_cop1Handler().
>
> Better fix is probably to modify fpu emulator so that it is preemption
> safe overall.
>
> Jun
>  
Missed this one ! I had a patch that enables preemption before the 
cond_resched and disables right after it. I forgot to send it to 
linux-mips though. But, I needed it to work on fpu-less CPU. My bad.
Thanks
Manish Lachwani