[PATCH] [ARM] Do not call flush_cache_user_range with mmap_sem held

[Russell King - ARM Linux]

On Thu, Apr 29, 2010 at 07:16:36PM +0100, Jamie Lokier wrote:
> Russell King - ARM Linux wrote:
> > On Wed, Apr 28, 2010 at 12:32:11AM -0700, Dima Zavin wrote:
> > > We can't be holding the mmap_sem while calling flush_cache_user_range
> > > because the flush can fault. If we fault on a user address, the
> > > page fault handler will try to take mmap_sem again. Since both places
> > > acquire the read lock, most of the time it succeeds. However, if another
> > > thread tries to acquire the write lock on the mmap_sem (e.g. mmap) in
> > > between the call to flush_cache_user_range and the fault, the down_read
> > > in do_page_fault will deadlock.
> > 
> > That's a non-issue.  If do_cache_op is holding a read lock, _nothing_
> > _else_ can be holding that lock in write mode.  So, holding the lock in
> > read mode ensures that when faults occur, the fault handler can be sure
> > that its read lock will succeed.
> 
> read-write locks will block a reader when there is a already blocked
> writer.  Otherwise the writer can be permanently starved due to new
> readers always arriving so reader count doesn't reach zero.

Hmm, true, and rather unfortunate.

As I've already said, we can not do this cache maintainence outside of
the lock.

The point of this code is to first validate that the region we're working
on is valid.  As soon as we drop the lock, we lose the guarantee that
the region is valid while we operate on it - indeed, the region could be
unmapped and remapped by a different thread.

I think the only reasonable solution is to also walk the page tables
and do the cache handling on a per-page basis, which will make this
interface quite a bit slower - but that's the price we pay for
correctness.