Re: [PATCH] 2.5.1-pre5: per-cpu areas

[Richard Henderson]

On Thu, Mar 14, 2002 at 03:37:38PM +1100, Rusty Russell wrote:
> > I am also a bit concerned however about aliasing that the compiler
> > might not detect.  For example, with this code:
> > 
> > 	this_cpu(foo) = 13;
> > 	per_cpu(foo, 0) = 15;
> > 	printf("foo=%d\n", this_cpu(foo);
> > 
> > might print the wrong value if gcc thinks that the first and second
> > assignment never alias each other.  Does HIDE_RELOC() take care of
> > this also?
> 
> I'd be pretty sure the compiler can't assume that.  Richard would
> know...

I can't think of a way your current code is invalid.  It's all
hidden behind an asm.  The compiler could guess the two addresses
are the same iff smp_processor_id() is the constant 0, aka UP.

> > On a side-note, would you mind moving __per_cpu_data from smp.h into
> > compiler.h?  I'd like to use it in processor.h and from that file, I
> > can't include smp.h due to a recursive dependency.

This definitely needs to be per-architecture.  On Alpha, I think I
can use the Thread Local Storage model to be added to binutils 2.13
(and potentially compiler support to gcc 3.[23]).  IA-64 may be able
to do the same.  It's certain that x86 can't, since the userland
model requires %gs:0 point to the thread base, and the kernel folk
would never cotton to the segment swapping that would be needed.

r~