• Peter Hurley's avatar
    tty: Make ldisc input flow control concurrency-friendly · 24a89d1c
    Peter Hurley authored
    Although line discipline receiving is single-producer/single-consumer,
    using tty->receive_room to manage flow control creates unnecessary
    critical regions requiring additional lock use.
    Instead, introduce the optional .receive_buf2() ldisc method which
    returns the # of bytes actually received. Serialization is guaranteed
    by the caller.
    In turn, the line discipline should schedule the buffer work item
    whenever space becomes available; ie., when there is room to receive
    data and receive_room() previously returned 0 (the buffer work
    item stops processing if receive_buf2() returns 0). Note the
    'no room' state need not be atomic despite concurrent use by two
    threads because only the buffer work thread can set the state and
    only the read() thread can clear the state.
    Add n_tty_receive_buf2() as the receive_buf2() method for N_TTY.
    Provide a public helper function, tty_ldisc_receive_buf(), to use
    when directly accessing the receive_buf() methods.
    Line disciplines not using input flow control can continue to set
    tty->receive_room to a fixed value and only provide the receive_buf()
    Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
tty_ldisc.h 7.79 KB