lockless tty flip buffers

** v2 changes **
- Rebased on v4 of 'lockless n_tty receive path'

This 2nd of 4 patchsets implements lockless receive from the tty driver. By lockless, I'm referring to the 'lock' spin lock formerly used to serialize access to the flip buffer list.

Since the driver-side flip buffer usage is already single-threaded and line discipline receiving is already single-threaded, implementing a lockless flip buffer list was the primary hurdle. [The only 2 flip buffer consumers, flush_to_ldisc() and tty_buffer_flush() were already mutually exclusive and this exclusion remains although the mechanism is changed.]

Since the flip buffer consumers, flush_to_ldisc() and tty_buffer_flush(), already leave the last-consumed flip buffer on the list, and since the existing flip buffer api is already divided into an add/commit interface, most of the requirement for a lockless algorithm was already in-place. The main differences are;
1) the initial state of the flip buffer list points head and tail to a 0-sized sentinel flip buffer. This eliminates head & tail NULL testing and assigning the head ptr from the driver-side thread. This sentinel is 'consumed' on the first iteration of ldisc receiving and does not require special-case logic.
2) the free list uses the atomic singly-linked llist interface. While this guarantees safe concurrent usage by both producer and consumer, it's not optimal. Both producer and consumer unnecessarily contend over the free list head ptr; a better approach would be to maintain an unconsumed buffer in the same way the flip buffer list works. Light testing has shown this contention accounts for roughly 5% of total cpu time in end-to-end copying.
3) The mutual exclusion between consumers is reimplemented as a mutex; this eliminates the need to drop the lock across the ldisc receive_buf() method. This mutual exclusion is extended to a public interface which the vt driver now uses to safely utilize the ldisc receive_buf() interface when pasting a selection.

Peter Hurley (16):
tty: Compute flip buffer ptrs
tty: Fix flip buffer free list
tty: Factor flip buffer initialization into helper function
tty: Merge tty_buffer_find() into tty_buffer_alloc()
tty: Use generic names for flip buffer list cursors
tty: Use lockless flip buffer free list
tty: Simplify flip buffer list with 0-sized sentinel
tty: Track flip buffer memory limit atomically
tty: Make driver-side flip buffers lockless
tty: Ensure single-threaded flip buffer consumer with mutex
tty: Only perform flip buffer flush from tty_buffer_flush()
tty: Avoid false-sharing flip buffer ptrs
tty: Use non-atomic state to signal flip buffer flush pending
tty: Merge __tty_flush_buffer() into lone call site
tty: Fix unsafe vt paste_selection()
tty: Remove private constant from global namespace

 drivers/staging/dgrp/dgrp_tty.c |   2 +
 drivers/tty/pty.c               |  10 +-
 drivers/tty/tty_buffer.c        | 395 +++++++++++++++++++---------------------
 drivers/tty/vt/selection.c      |   4 +-
 include/linux/llist.h           |  23 +++
 include/linux/tty.h             |  39 ++--
 include/linux/tty_flip.h        |   8 +-
 7 files changed, 244 insertions(+), 237 deletions(-)

1.8.1.2