Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT

[Luis R. Rodriguez]

On Wed, Dec 03, 2014 at 08:39:47PM +0100, Luis R. Rodriguez wrote:
> On Wed, Dec 03, 2014 at 05:37:51AM +0100, Juergen Gross wrote:
> > On 12/03/2014 03:28 AM, Luis R. Rodriguez wrote:
> >> On Tue, Dec 02, 2014 at 11:11:18AM +0000, David Vrabel wrote:
> >>> On 01/12/14 22:36, Luis R. Rodriguez wrote:
> >>>>
> >>>> Then I do agree its a fair analogy (and find this obviously odd that how
> >>>> widespread cond_resched() is), we just don't have an equivalent for IRQ
> >>>> context, why not avoid the special check then and use this all the time in the
> >>>> middle of a hypercall on the return from an interrupt (e.g., the timer
> >>>> interrupt)?
> >>>
> >>> http://lists.xen.org/archives/html/xen-devel/2014-02/msg01101.html
> >>
> >> OK thanks! That explains why we need some asm code but in that submission you
> >> still also had used is_preemptible_hypercall(regs) and in the new
> >> implementation you use a CPU variable xen_in_preemptible_hcall prior to calling
> >> preempt_schedule_irq(). I believe you added the CPU variable because
> >> preempt_schedule_irq() will preempt first without any checks if it should, I'm
> >> asking why not do something like cond_resched_irq() where we check with
> >> should_resched() prior to preempting and that way we can avoid having to use
> >> the CPU variable?
> >
> > Because that could preempt at any asynchronous interrupt making the
> > no-preempt kernel fully preemptive. 
> 
> OK yeah I see. That still doesn't negate the value of using something
> like cond_resched_irq() with a should_resched() on only critical hypercalls.
> The current implementation (patch by David) forces preemption without
> checking for should_resched() so it would preempt unnecessarily at least
> once.
> 
> > How would you know you are just
> > doing a critical hypercall which should be preempted?
> 
> You would not, you're right. I was just trying to see if we could generalize
> an API for this to avoid having users having to create their own CPU variables
> but this all seems very specialized as we want to use this on the timer
> so if we do generalize a cond_resched_irq() perhaps the documentation can
> warn about this type of case or abuse.

David's patch had the check only it was x86 based, if we use cond_resched_irq()
we can leave that aspect out to be done through asm inlines or it'll use the
generic shoudl_resched(), that should save some code on the asm implementations.

I have some patches now which generalizees this, I also have more information
about this can happen exactly, and a way to triggger it on small systems with
some hacks to emulate possibly backend behaviour on larger systems. In the worst
case this can be a dangerious situation to be in. I'll send some new RFTs.

 Luis
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