Re: [PATCH v2 00/18] lockless n_tty receive path

[Peter Hurley]

On Wed, 2013-03-27 at 07:43 -0400, Peter Hurley wrote:
> ** 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.

And adds Ilya's suggestions for cleaning up 'tty: Simplify tty
buffer/ldisc interface with helper function'

> 
> 
> 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(-)
> 


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