Re: [PATCH for 2.5] preemptible kernel

Andrea Arcangeli
2001-03-21T15:46:22Z

On Wed, Mar 21, 2001 at 08:19:54PM +1100, Keith Owens wrote:
> Ouch. What about all the per cpu structures in the kernel, how do you
> handle them if a preempted task can be rescheduled on another cpu?
>
> int count[NR_CPUS], *p;
> p = count+smp_processor_id(); /* start on cpu 0, &count[0] */
> if (*p >= 1024) {
> /* preempt here, reschedule on cpu 1 */
> *p = 1; /* update cpu 0 count from cpu 1, oops */
> }
>
> Unless you find every use of a per cpu structure and wrap a spin lock
> around it, migrating a task from one cpu to another is going to be a
> source of wierd and wonderful errors. Since the main use of per cpu
> structures is to avoid locks, adding a spin lock to every structure
> will be a killer. Or have I missed something?

That's why Linus suggested it for UP only.

Andrea