On Fri, 16 May 2014, Tejun Heo wrote:
> On Fri, May 16, 2014 at 01:57:37PM +0200, Peter Zijlstra wrote:
> > This of course leaves the question how the workqueue code manages to
> > call set_cpu_allowed_ptr() on a cpu _before_ its online.
> >
> > That too sounds fishy.. with the proposed patch the
> > set_cpus_allowed_ptr() will 'gracefully' fail, but calling it in the
> > first place is of course dubious too.
>
> Right after being created, a workqueue worker invokes
> set_cpus_allowed_ptr() to the target cpumask without checking whether
> the cpu[s] are online or not and it's allowed to fail. The guarantee
> there is that the worker is already registered by that point and if a
> CPU comes online after the registration, CPU_ONLINE notification will
> update the cpumask accordingly, so either way the worker is guaranteed
> to be on the right cpumask.
That's what the kthread_create_on_cpu/kthread_park/kthread_unpark
infrastructure is for.
Thanks,
tglx
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