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 01:19:50PM +0100, 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?
> 
> > +
> > +   ret = func(par);
> > +
> > +   set_cpus_allowed_ptr(current, old_mask);
> > +
> > +out:
> > +   free_cpumask_var(old_mask);
> > +   return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(call_sync_on_phys_cpu);
> 
> This is disgusting, and you're adding this to !Xen kernels too.

how about something like:

struct xen_callback_struct {
        struct work_struct      work;
        struct completion       done;
        void *                  data;
        int                     ret;
};

static void xen_callback_f(struct work_struct *work)
{
        struct xen_callback_struct *xcs = container_of(work, struct 
xen_callback_struct, work);

        xcs->ret = xcs->func(xcs->data);

        complete(&xcs->done);
}

xen_call_on_cpu_sync(int cpu, int (*func)(void *), void *data)
{
        struct xen_callback_state xcs = {
                .work = __WORK_INITIALIZER(xcs.work, xen_callback_f);
                .done = COMPLETION_INITIALIZER_ONSTACK(xcs.done),
                .data = data,
        };

        queue_work_on(&work, cpu);
        wait_for_completion(&xcs.done);

        return xcs.ret;
}

No mucking about with the scheduler state, no new exported functions
etc..

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