On Thu, 8 May 2008 00:44:06 +0200 Andrea Arcangeli <andrea@xxxxxxxxxxxx> wrote: > On Wed, May 07, 2008 at 03:31:03PM -0700, Andrew Morton wrote: > > Nope. We only need to take the global lock before taking *two or more* of > > the per-vma locks. > > > > I really wish I'd thought of that. > > I don't see how you can avoid taking the system-wide-global lock > before every single anon_vma->lock/i_mmap_lock out there without > mm_lock. > > Please note, we can't allow a thread to be in the middle of > zap_page_range while mmu_notifier_register runs. > > vmtruncate takes 1 single lock, the i_mmap_lock of the inode. Not more > than one lock and we've to still take the global-system-wide lock > _before_ this single i_mmap_lock and no other lock at all. > > Please elaborate, thanks! umm... CPU0: CPU1: spin_lock(a->lock); spin_lock(b->lock); spin_lock(b->lock); spin_lock(a->lock); bad. CPU0: CPU1: spin_lock(global_lock) spin_lock(global_lock); spin_lock(a->lock); spin_lock(b->lock); spin_lock(b->lock); spin_lock(a->lock); Is OK. CPU0: CPU1: spin_lock(global_lock) spin_lock(a->lock); spin_lock(b->lock); spin_lock(b->lock); spin_unlock(b->lock); spin_lock(a->lock); spin_unlock(a->lock); also OK. As long as all code paths which can take two-or-more locks are all covered by the global lock there is no deadlock scenario. If a thread takes just a single instance of one of these locks without taking the global_lock then there is also no deadlock. Now, if we need to take both anon_vma->lock AND i_mmap_lock in the newly added mm_lock() thing and we also take both those locks at the same time in regular code, we're probably screwed. ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. _______________________________________________ kvm-devel mailing list kvm-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/kvm-devel