Re: [Xen-devel] [PATCH 2/6] sched: add function to execute a function synchronously on a physical cpu

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, Mar 11, 2016 at 01:43:53PM +0100, Juergen Gross wrote:
> On 11/03/16 13:19, Peter Zijlstra wrote:
> > On Fri, Mar 11, 2016 at 12:59:30PM +0100, Juergen Gross wrote:
> >> +int call_sync_on_phys_cpu(unsigned cpu, int (*func)(void *), void *par)
> >> +{
> >> +  cpumask_var_t old_mask;
> >> +  int ret;
> >> +
> >> +  if (cpu >= nr_cpu_ids)
> >> +          return -EINVAL;
> >> +
> >> +  if (!alloc_cpumask_var(&old_mask, GFP_KERNEL))
> >> +          return -ENOMEM;
> >> +
> >> +  cpumask_copy(old_mask, &current->cpus_allowed);
> >> +  ret = set_cpus_allowed_ptr(current, cpumask_of(cpu));
> >> +  if (ret)
> >> +          goto out;
> > 
> > So what happens if someone does sched_setaffinity() right about here?
> 
> Aah, okay. Seems I should add preempt_disable() in this patch already
> and retry the set_cpus_allowed_ptr() in case cpus_allowed has changed.

Doesn't help, you should simply not _ever_ touch this for random tasks.

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