On Fri, Aug 27, 2010 at 10:13 AM, Christoph Lameter <cl@xxxxxxxxx> wrote: > The basic problem with SLAB_DESTROY_BY_RCU is that you get a reference to > an object that is guaranteed only to have the same type (the instance may > fluctuate and be replaced from under you unless other measures are taken). (I wouldn't describe that as a "problem with SLAB_DESTROY_BY_RCU": it's precisely the nature of SLAB_DESTROY_BY_RCU, what makes it useful in solving backward-locking problems elsewhere.) > > Typically one must take a lock within the memory structure to pin down > the object (or take a refcount). Only then can you follow pointers and > such. It is only possible to verify that the right object has been > reached *after* locking. Following a pointer without having determined > that we hit the right object should not occur. > > A solution here would be to take the anon_vma->lock (prevents the object > switching under us) and then verify that the mapping is the one we are > looking for and that the pointer points to the right root. Then take the > root lock. > > Hughs solution takes a global spinlock which will limit scalability. Eh? My solution was a second page_mapped(page) test i.e. testing an atomic. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>