** 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 -- 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
