Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

[Rusty Russell <rusty@rustcorp.com.au>]

On Tuesday 10 June 2008 05:00:36 Christoph Lameter wrote:
> On Fri, 6 Jun 2008, Rusty Russell wrote:
> > > Also, the above cpu_local_wrap(...) adds:
> > >
> > > 	#define cpu_local_wrap(l)               \
> > > 	({                                      \
> > > 	        preempt_disable();              \
> > > 	        (l);                            \
> > > 	        preempt_enable();               \
> > > 	})                                      \
> > >
> > > ... and there isn't a non-preemption version that I can find.
> >
> > Yes, this should be fixed.  I thought i386 had optimized versions
> > pre-merge, but I was wrong (%gs for per-cpu came later, and noone cleaned
> > up these naive versions).  Did you want me to write them?
>
> How can that be fixed? You have no atomic instruction that calculates the
> per cpu address in one go.

Huh?  "incl %fs:varname" does exactly this.

> And as long as that is the case you need to 
> disable preempt. Otherwise you may increment the per cpu variable of
> another processor because the process was rescheduled after the address
> was calculated but before the increment was done.

But of course, that is not a problem.  You make local_t an atomic_t, and then 
it doesn't matter which CPU you incremented.

By definition if the caller cared, they would have had premption disabled.

Hope that clarifies,
Rusty.