Some futex functions do get_user calls while holding mmap_sem for
reading.  If get_user() faults, and another thread happens to be in mmap
(or somewhere else holding waiting on down_write for the same
semaphore), then do_page_fault will deadlock.  Most architectures seem
to be exposed to this.

To avoid it, make sure the page is available.  If not, release the
semaphore, fault it in and retry.

I also found another exposure by inspection, moving some of the code
around avoids the possible deadlock there.
Signed-off-by: Olof Johansson <olof@austin.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>