Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 16, 2009 at 10:40:55AM -0400, Gregory Haskins wrote:
> > This is ugly, isn't it? With CONFIG_PREEMPT=no preemptible() is always false.
> >
> > Further, to do useful things it might not be enough that you can sleep:
> > with iofd you also want to access current task with e.g. copy from user.
> >   
> 
> Something else to consider:  For iosignalfd, we can assume we will
> always be called from vcpu process context so we might not really need
> official affirmation from the system.  For irqfd, we cannot predict who
> may be injecting the interrupt (for instance, it might be a
> PCI-passthrough hard-irq context).  I am not sure if this knowledge
> actually helps to simplify the problem space, but I thought I should
> mention it.
> 
> -Greg
> 
> 

The way this is addressed with eventfd_signal_task proposal is:
- user calls eventfd_signal_task
  we look at current->mm and figure out whether this is the right
  context or we need a context switch
- everyone else calls eventfd_signal
  we know that we need a context switch