futex: Fix regression with read only mappings

commit 7485d0d3

 (futexes: Remove rw
parameter from get_futex_key()) in 2.6.33 fixed two problems:  First, It
prevented a loop when encountering a ZERO_PAGE. Second, it fixed RW
MAP_PRIVATE futex operations by forcing the COW to occur by
unconditionally performing a write access get_user_pages_fast() to get
the page.  The commit also introduced a user-mode regression in that it
broke futex operations on read-only memory maps.  For example, this
breaks workloads that have one or more reader processes doing a
FUTEX_WAIT on a futex within a read only shared file mapping, and a
writer processes that has a writable mapping issuing the FUTEX_WAKE.

This fixes the regression for valid futex operations on RO mappings by
trying a RO get_user_pages_fast() when the RW get_user_pages_fast()
fails. This change makes it necessary to also check for invalid use
cases, such as anonymous RO mappings (which can never change) and the
ZERO_PAGE which the commit referenced above was written to address.

This patch does restore the original behavior with RO MAP_PRIVATE
mappings, which have inherent user-mode usage problems and don't really
make sense.  With this patch performing a FUTEX_WAIT within a RO
MAP_PRIVATE mapping will be successfully woken provided another process
updates the region of the underlying mapped file.  However, the mmap()
man page states that for a MAP_PRIVATE mapping:

  It is unspecified whether changes made to the file after
  the mmap() call are visible in the mapped region.

So user-mode users attempting to use futex operations on RO MAP_PRIVATE
mappings are depending on unspecified behavior.  Additionally a
RO MAP_PRIVATE mapping could fail to wake up in the following case.

  Thread-A: call futex(FUTEX_WAIT, memory-region-A).
            get_futex_key() return inode based key.
            sleep on the key
  Thread-B: call mprotect(PROT_READ|PROT_WRITE, memory-region-A)
  Thread-B: write memory-region-A.
            COW happen. This process's memory-region-A become related
            to new COWed private (ie PageAnon=1) page.
  Thread-B: call futex(FUETX_WAKE, memory-region-A).
            get_futex_key() return mm based key.
            IOW, we fail to wake up Thread-A.

Once again doing something like this is just silly and users who do
something like this get what they deserve.

While RO MAP_PRIVATE mappings are nonsensical, checking for a private
mapping requires walking the vmas and was deemed too costly to avoid a
userspace hang.

This Patch is based on Peter Zijlstra's initial patch with modifications to
only allow RO mappings for futex operations that need VERIFY_READ access.

Reported-by: David Oliver <david@rgmadvisors.com>
Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: peterz@infradead.org
Cc: eric.dumazet@gmail.com
Cc: zvonler@rgmadvisors.com
Cc: hughd@google.com
Link: http://lkml.kernel.org/r/1309450892-30676-1-git-send-email-sbohrer@rgmadvisors.com
Cc: stable@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>