The Lockronomicon Your guide to the ancient and twisted locking policies of the tty layer and the warped logic behind them. Beware all ye who read on. FIXME: still need to work out the full set of BKL assumptions and document them so they can eventually be killed off. Line Discipline --------------- Line disciplines are registered with tty_register_ldisc() passing the discipline number and the ldisc structure. At the point of registration the discipline must be ready to use and it is possible it will get used before the call returns success. If the call returns an error then it won't get called. Do not re-use ldisc numbers as they are part of the userspace ABI and writing over an existing ldisc will cause demons to eat your computer. After the return the ldisc data has been copied so you may free your own copy of the structure. You must not re-register over the top of the line discipline even with the same data or your computer again will be eaten by demons. In order to remove a line discipline call tty_register_ldisc passing NULL. In ancient times this always worked. In modern times the function will return -EBUSY if the ldisc is currently in use. Since the ldisc referencing code manages the module counts this should not usually be a concern. Heed this warning: the reference count field of the registered copies of the tty_ldisc structure in the ldisc table counts the number of lines using this discipline. The reference count of the tty_ldisc structure within a tty counts the number of active users of the ldisc at this instant. In effect it counts the number of threads of execution within an ldisc method (plus those about to enter and exit although this detail matters not). Line Discipline Methods ----------------------- TTY side interfaces: close() - This is called on a terminal when the line discipline is being unplugged. At the point of execution no further users will enter the ldisc code for this tty. Can sleep. open() - Called when the line discipline is attached to the terminal. No other call into the line discipline for this tty will occur until it completes successfully. Can sleep. write() - A process is writing data from user space through the line discipline. Multiple write calls are serialized by the tty layer for the ldisc. May sleep. flush_buffer() - May be called at any point between open and close. chars_in_buffer() - Report the number of bytes in the buffer. set_termios() - Called on termios structure changes. The caller passes the old termios data and the current data is in the tty. Currently can be parallel entered and ordering isn't predictable - FIXME read() - Move data from the line discipline to the user. Multiple read calls may occur in parallel and the ldisc must deal with serialization issues. May sleep. poll() - Check the status for the poll/select calls. Multiple poll calls may occur in parallel. May sleep. ioctl() - Called when an ioctl is handed to the tty layer that might be for the ldisc. Multiple ioctl calls may occur in parallel. May sleep. Driver Side Interfaces: receive_buf() - Hand buffers of bytes from the driver to the ldisc for processing. Semantics currently rather mysterious 8( receive_room() - Can be called by the driver layer at any time when the ldisc is opened. The ldisc must be able to handle the reported amount of data at that instant. Synchronization between active receive_buf and receive_room calls is down to the driver not the ldisc. Must not sleep. write_wakeup() - May be called at any point between open and close. The TTY_DO_WRITE_WAKEUP flag indicates if a call is needed but always races versus calls. Thus the ldisc must be careful about setting order and to handle unexpected calls. Must not sleep. Locking Callers to the line discipline functions from the tty layer are required to take line discipline locks. The same is true of calls from the driver side but not yet enforced. Three calls are now provided ldisc = tty_ldisc_ref(tty); takes a handle to the line discipline in the tty and returns it. If no ldisc is currently attached or the ldisc is being closed and re-opened at this point then NULL is returned. While this handle is held the ldisc will not change or go away. tty_ldisc_deref(ldisc) Returns the ldisc reference and allows the ldisc to be closed. Returning the reference takes away your right to call the ldisc functions until you take a new reference. ldisc = tty_ldisc_ref_wait(tty); Performs the same function as tty_ldisc_ref except that it will wait for an ldisc change to complete and then return a reference to the new ldisc. While these functions are slightly slower than the old code they should have minimal impact as most receive logic uses the flip buffers and they only need to take a reference when they push bits up through the driver. A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc functions are called with the ldisc unavailable. Thus tty_ldisc_ref will fail in this situation if used within these functions. Ldisc and driver code calling its own functions must be careful in this case. Driver Interface ---------------- open() - Called when a device is opened. May sleep close() - Called when a device is closed. At the point of return from this call the driver must make no further ldisc calls of any kind. May sleep write() - Called to write bytes to the device. May not sleep. May occur in parallel in special cases. Because this includes panic paths drivers generally shouldn't try and do clever locking here. put_char() - Stuff a single character onto the queue. The driver is guaranteed following up calls to flush_chars. flush_chars() - Ask the kernel to write put_char queue write_room() - Return the number of characters tht can be stuffed into the port buffers without overflow (or less). The ldisc is responsible for being intelligent about multi-threading of write_room/write calls ioctl() - Called when an ioctl may be for the driver set_termios() - Called on termios change, serialized against itself by a semaphore. May sleep. set_ldisc() - Notifier for discipline change. At the point this is done the discipline is not yet usable. Can now sleep (I think) throttle() - Called by the ldisc to ask the driver to do flow control. Serialization including with unthrottle is the job of the ldisc layer. unthrottle() - Called by the ldisc to ask the driver to stop flow control. stop() - Ldisc notifier to the driver to stop output. As with throttle the serializations with start() are down to the ldisc layer. start() - Ldisc notifier to the driver to start output. hangup() - Ask the tty driver to cause a hangup initiated from the host side. [Can sleep ??] break_ctl() - Send RS232 break. Can sleep. Can get called in parallel, driver must serialize (for now), and with write calls. wait_until_sent() - Wait for characters to exit the hardware queue of the driver. Can sleep send_xchar() - Send XON/XOFF and if possible jump the queue with it in order to get fast flow control responses. Cannot sleep ??