Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to call_rcu in vhost_set_memory() in vhost kernel module?

[Paolo Bonzini]

Il 12/05/2014 14:53, Michael S. Tsirkin ha scritto:
> > > In any case, whether writes synchronize with RCU or bypass it
> > > doesn't change the picture.  In either case, writes are ordered
> > > against each other but not against reads.  RCU does nothing except
> > > preventing dangling pointer accesses.
> > >
> > > Paolo
> This is the only part I don't get.
> RCU will make sure no VCPUs are running, won't it?
> So it's a kind of full barrier.

The actual point where the new value becomes visible is where the write 
happens, not where you do synchronize_rcu.  This is the same for both 
full-copy of the routing table or overwriting the entry.

However, the delay in servicing an older irqfd write can be arbitrary if 
the scheduler decides not to run the irqfd_inject thread.  Such an older 
write might definitely read a newer routing entry.  And that's the 
invalid scenario according to the PCI spec.

Paolo