Matt Mackall wrote:

On Tue, Mar 20, 2007 at 09:31:58AM -0700, Jeremy Fitzhardinge wrote:
  

quoted

Linus Torvalds wrote:
    

quoted

On Tue, 20 Mar 2007, Eric W. Biederman wrote:
  
      

quoted

If that is the case.  In the normal kernel what would
the "the oops, we got an interrupt code do?"
I assume it would leave interrupts disabled when it returns?
Like we currently do with the delayed disable of normal interrupts?
    
        

Yeah, disable interrupts, and set a flag that the fake "sti" can test, and 
just return without doing anything.

(You may or may not also need to do extra work to Ack the hardware 
interrupt etc, which may be irq-controller specific. Once the CPU has 
accepted the interrupt, you may not be able to just leave it dangling)
  
      

So it would be something like:

    pda.intr_mask = 1;		/* disable interrupts */
    ...
    pda.intr_mask = 0;		/* enable interrupts */
    if (xchg(&pda.intr_pending, 0))	/* check pending */
    	asm("sti");		/* was pending; isr left cpu interrupts masked */
    

I don't know that you need an xchg there. If you're still on the same
CPU, it should all be nice and causal even across an interrupt handler.
So it could be:

   pda.intr_mask = 0; /* intr_pending can't get set after this */
   if (unlikely(pda.intr_pending)) {
      pda.intr_pending = 0;
      asm("sti");
   }

(This would actually need a C barrier, but I'll ignore that as this'd
end up being asm...)

But other interesting things could happen. If we never did a real CLI
and we get preempted and switched to another CPU between clearing
intr_mask and checking intr_pending, we get a little confused. 
  

Could prevent preempt if pda.intr_mask is set.  preemptible() is defined as:

    # define preemptible()    (preempt_count() == 0 && !irqs_disabled())

anyway, so that would be changed to look at the intr_mask rather than
eflags.
(I'm not sure if preemptible() is actually used to determine whether
preempt or not).

Alternatively, the intr_mask could be encoded in a bit of preempt_count...

    J