Re: [PATCH 08 of 11] anon-vma-rwsem

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 8 May 2008 00:44:06 +0200
Andrea Arcangeli <andrea@qumranet.com> 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.