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Gmane
From: Peter Hurley <peter <at> hurleysoftware.com>
Subject: [PATCH v2 00/18] lockless n_tty receive path
Newsgroups: gmane.linux.kernel
Date: Wednesday 27th March 2013 11:43:50 UTC (over 3 years ago)
** v2 changes **
- Rebased on top of 'tty: Fix race condition if flushing tty flip buffers'
- I forgot to mention; this is ~35% faster on end-to-end tests on SMP.



This patchset implements lockless receive from tty flip buffers
to the n_tty read buffer and lockless copy into the user-space
read buffer.

By lockless, I'm referring to the fine-grained read_lock formerly used
to serialize access to the shared n_tty read buffer (which wasn't being
used everywhere it should have been).

In the current n_tty, the read_lock is grabbed a minimum of
3 times per byte!
- ^^^^
- should say 2 times per byte!

The read_lock is unnecessary to serialize access between the flip
buffer work and the single reader, as this is a
single-producer/single-consumer pattern.

However, other threads may attempt to read or modify the buffer indices,
notably for buffer flushing and for setting/resetting termios
(there are some others). In addition, termios changes can cause
havoc while the tty flip buffer work is pushing more data.
Read more about that here: https://lkml.org/lkml/2013/2/22/480

Both hurdles are overcome with the same mechanism: converting the
termios_mutex to a r/w semaphore (just a normal one :).

Both the receive_buf() path and the read() path claim a reader lock
on the termios_rwsem. This prevents concurrent changes to termios.
Also, flush_buffer() and TIOCINQ ioctl obtain a write lock on the
termios_rwsem to exclude the flip buffer work and user-space read
from accessing the buffer indices while resetting them.

This patchset also implements a block copy from the read_buf
into the user-space buffer in canonical mode (rather than the
current byte-by-byte method).



Greg,

Unfortunately, this series is dependent on the 'ldsem patchset'.
The reason is that this series abandons tty->receive_room as
a flow control mechanism (because that requires locking),
and the TIOCSETD ioctl _without ldsem_ uses tty->receive_room
to shutoff i/o.


Peter Hurley (18):
  tty: Don't change receive_room for ioctl(TIOCSETD)
  tty: Make ldisc input flow control concurrency-friendly
  tty: Simplify tty buffer/ldisc interface with helper function
  n_tty: Factor canonical mode copy from n_tty_read()
  n_tty: Line copy to user buffer in canonical mode
  n_tty: Split n_tty_chars_in_buffer() for reader-only interface
  tty: Deprecate ldisc .chars_in_buffer() method
  n_tty: Get read_cnt through accessor
  n_tty: Don't wrap input buffer indices at buffer size
  n_tty: Remove read_cnt
  tty: Convert termios_mutex to termios_rwsem
  n_tty: Access termios values safely
  n_tty: Replace canon_data with index comparison
  n_tty: Make N_TTY ldisc receive path lockless
  n_tty: Reset lnext if canonical mode changes
  n_tty: Fix type mismatches in receive_buf raw copy
  n_tty: Don't wait for buffer work in read() loop
  n_tty: Separate buffer indices to prevent cache-line sharing

 drivers/net/irda/irtty-sir.c |   8 +-
 drivers/tty/n_tty.c          | 550
+++++++++++++++++++++++++++----------------
 drivers/tty/pty.c            |   4 +-
 drivers/tty/tty_buffer.c     |  33 ++-
 drivers/tty/tty_io.c         |  14 +-
 drivers/tty/tty_ioctl.c      |  90 +++----
 drivers/tty/tty_ldisc.c      |  13 +-
 drivers/tty/vt/vt.c          |   4 +-
 include/linux/tty.h          |   7 +-
 include/linux/tty_ldisc.h    |   8 +
 10 files changed, 439 insertions(+), 292 deletions(-)

-- 
1.8.1.2

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