Re: [RFC] 4KB stack + irq stack for x86

[Linus Torvalds]

On 5 Jun 2002, Andi Kleen wrote:
> > 
> > Ah, you're right.  If anyone uses current_thread_info from IRQ context 
> > it will set the flags in the wrong structure.  However, it actually 
> > works because nobody does that currently: all of the _thread_flag users 
> 
> preemptive kernels do use current_thread_info() for every spinlock.
> this required me to change its implementation on x86-64 from stack
> arithmetic to access the base register. 

Note that this part is ok, as long as we make sure that the irq stack gets
initialized with a preempt_count > 0 (we must not preempt an interrupt
handler anyway, it wouldn't work), _and_ we make sure that taking the
interrupt also increments the "process native" preempt_count (so that
anybody looking at that preempt_count to determine whether it could be
preempted will also get a "nope, don't preempt me").

So that part doesn't look like a fundamental problem to me. It's just a 
"need to be careful" thing.

		Linus