On Sun, Apr 06, 2008 at 10:48:56PM -0700, Christoph Lameter wrote: > On Sat, 5 Apr 2008, Andrea Arcangeli wrote: > > > > + rcu_assign_pointer(mm->emm_notifier, e); > > > + mm_unlock(mm); > > > > My mm_lock solution makes all rcu serialization an unnecessary > > overhead so you should remove it like I already did in #v11. If it > > wasn't the case, then mm_lock wouldn't be a definitive fix for the > > race. > > There still could be junk in the cache of one cpu. If you just read the > new pointer but use the earlier content pointed to then you have a > problem.
There can't be junk, spinlocks provides semantics of proper memory barriers, just like rcu, so it's entirely superflous. There could be junk only if any of the mmu_notifier_* methods would be invoked _outside_ the i_mmap_lock and _outside_ the anon_vma and outside the mmap_sem, that is never the case of course. > So a memory fence / barrier is needed to guarantee that the contents > pointed to are fetched after the pointer. It's not needed... if you were right we could never possibly run a list_for_each inside any spinlock protected critical section and we'd always need to use the _rcu version instead. The _rcu version is needed only when the list walk happens outside the spinlock critical section of course (rcu = no spinlock cacheline exlusive write operation in the read side, here the read side takes the spinlock big time). ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Register now and save $200. Hurry, offer ends at 11:59 p.m., Monday, April 7! Use priority code J8TLD2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
