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(-) > -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
