On Fri, 7 Mar 2008, Andrea Arcangeli wrote: > In the meantime I've also been thinking that we could need the > write_seqlock in mmu_notifier_register, to know when to restart the > loop if somebody does a mmu_notifier_register; > synchronize_rcu(). Otherwise there's no way to be sure the mmu > notifier will start firing immediately after synchronize_rcu. I'm > unsure if it's acceptable that in-progress mmu notifier invocations, > don't need to notice the fact that somebody did mmu_notifier_register; > synchronize_rcu. If they don't need to notice, then we can just drop > unregister and all rcu_read_lock()s instead of adding write_seqlock to > the register operation.
This is all getting into some very complicated issues..... > Overall my effort is to try to avoid expand the list walk with > explicit memory barriers like in EMM while trying to be equally > efficient. The smp_rmb is such a big problem? You have seqlock, rcu etc all in there as well. I doubt that this is more efficient. > Another issue is that the _begin/_end logic doesn't provide any > guarantee that the _begin will start firing before _end, if a kernel > module is loaded while another cpu is already running inside some > munmap operation etc.. The KVM usage of mmu notifier has no problem > with that detail, but KVM doesn't use _begin at all, I wonder if > others would have problems. This is a kind of a separate problem, but > quite related to the question if the notifiers must be guaranteed to > start firing immediately after mmu_notifier_unregister;synchronize_rcu > or not, that's why I mentioned it here. Ahh. Yes that is an interesting issue. If a device driver cannot handle this then _begin must prohibit module loading. That means not allowing stop_machine_run I guess which should not be that difficult. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
