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! 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 | 26 +- 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, 437 insertions(+), 287 deletions(-) -- 1.8.1.2 -- 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
