[PATCH] tty layer comment the locking assumptions and functions somewhat

Doesn't fix them but does show up some interesting areas that need review
and fixing.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 658 insertions, 60 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index d6e4eaa48538..2cef982585f0 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -153,6 +153,15 @@ int tty_ioctl(struct inode * inode, struct file * file,
 static int tty_fasync(int fd, struct file * filp, int on);
 static void release_mem(struct tty_struct *tty, int idx);
 
+/**
+ *      alloc_tty_struct        -       allocate a tty object
+ *
+ *      Return a new empty tty structure. The data fields have not
+ *      been initialized in any way but has been zeroed
+ *
+ *      Locking: none
+ *      FIXME: use kzalloc
+ */
 
 static struct tty_struct *alloc_tty_struct(void)
 {
@@ -166,6 +175,15 @@ static struct tty_struct *alloc_tty_struct(void)
 
 static void tty_buffer_free_all(struct tty_struct *);
 
+/**
+ *      free_tty_struct         -       free a disused tty
+ *      @tty: tty struct to free
+ *
+ *      Free the write buffers, tty queue and tty memory itself.
+ *
+ *      Locking: none. Must be called after tty is definitely unused
+ */
+
 static inline void free_tty_struct(struct tty_struct *tty)
 {
         kfree(tty->write_buf);
@@ -175,6 +193,17 @@ static inline void free_tty_struct(struct tty_struct *tty)
 
 #define TTY_NUMBER(tty) ((tty)->index + (tty)->driver->name_base)
 
+/**
+ *      tty_name        -       return tty naming
+ *      @tty: tty structure
+ *      @buf: buffer for output
+ *
+ *      Convert a tty structure into a name. The name reflects the kernel
+ *      naming policy and if udev is in use may not reflect user space
+ *
+ *      Locking: none
+ */
+
 char *tty_name(struct tty_struct *tty, char *buf)
 {
         if (!tty) /* Hmm.  NULL pointer.  That's fun. */
@@ -235,6 +264,17 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
  * Tty buffer allocation management
  */
 
+
+/**
+ *      tty_buffer_free_all             -       free buffers used by a tty
+ *      @tty: tty to free from
+ *
+ *      Remove all the buffers pending on a tty whether queued with data
+ *      or in the free ring. Must be called when the tty is no longer in use
+ *
+ *      Locking: none
+ */
+
 static void tty_buffer_free_all(struct tty_struct *tty)
 {
         struct tty_buffer *thead;
@@ -347,6 +387,18 @@ int tty_buffer_request_room(struct tty_struct *tty, size_t size)
 }
 EXPORT_SYMBOL_GPL(tty_buffer_request_room);
 
+/**
+ *      tty_insert_flip_string  -       Add characters to the tty buffer
+ *      @tty: tty structure
+ *      @chars: characters
+ *      @size: size
+ *
+ *      Queue a series of bytes to the tty buffering. All the characters
+ *      passed are marked as without error. Returns the number added.
+ *
+ *      Locking: Called functions may take tty->buf.lock
+ */
+
 int tty_insert_flip_string(struct tty_struct *tty, const unsigned char *chars,
                                 size_t size)
 {
@@ -370,6 +422,20 @@ int tty_insert_flip_string(struct tty_struct *tty, const unsigned char *chars,
 }
 EXPORT_SYMBOL(tty_insert_flip_string);
 
+/**
+ *      tty_insert_flip_string_flags    -       Add characters to the tty buffer
+ *      @tty: tty structure
+ *      @chars: characters
+ *      @flags: flag bytes
+ *      @size: size
+ *
+ *      Queue a series of bytes to the tty buffering. For each character
+ *      the flags array indicates the status of the character. Returns the
+ *      number added.
+ *
+ *      Locking: Called functions may take tty->buf.lock
+ */
+
 int tty_insert_flip_string_flags(struct tty_struct *tty,
                 const unsigned char *chars, const char *flags, size_t size)
 {
@@ -394,6 +460,17 @@ int tty_insert_flip_string_flags(struct tty_struct *tty,
 }
 EXPORT_SYMBOL(tty_insert_flip_string_flags);
 
+/**
+ *      tty_schedule_flip       -       push characters to ldisc
+ *      @tty: tty to push from
+ *
+ *      Takes any pending buffers and transfers their ownership to the
+ *      ldisc side of the queue. It then schedules those characters for
+ *      processing by the line discipline.
+ *
+ *      Locking: Takes tty->buf.lock
+ */
+
 void tty_schedule_flip(struct tty_struct *tty)
 {
         unsigned long flags;
@@ -405,12 +482,19 @@ void tty_schedule_flip(struct tty_struct *tty)
 }
 EXPORT_SYMBOL(tty_schedule_flip);
 
-/*
+/**
+ *      tty_prepare_flip_string         -       make room for characters
+ *      @tty: tty
+ *      @chars: return pointer for character write area
+ *      @size: desired size
+ *
  *      Prepare a block of space in the buffer for data. Returns the length
  *      available and buffer pointer to the space which is now allocated and
  *      accounted for as ready for normal characters. This is used for drivers
  *      that need their own block copy routines into the buffer. There is no
  *      guarantee the buffer is a DMA target!
+ *
+ *      Locking: May call functions taking tty->buf.lock
  */
 
 int tty_prepare_flip_string(struct tty_struct *tty, unsigned char **chars, size_t size)
@@ -427,12 +511,20 @@ int tty_prepare_flip_string(struct tty_struct *tty, unsigned char **chars, size_
 
 EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
 
-/*
+/**
+ *      tty_prepare_flip_string_flags   -       make room for characters
+ *      @tty: tty
+ *      @chars: return pointer for character write area
+ *      @flags: return pointer for status flag write area
+ *      @size: desired size
+ *
  *      Prepare a block of space in the buffer for data. Returns the length
  *      available and buffer pointer to the space which is now allocated and
  *      accounted for as ready for characters. This is used for drivers
  *      that need their own block copy routines into the buffer. There is no
  *      guarantee the buffer is a DMA target!
+ *
+ *      Locking: May call functions taking tty->buf.lock
  */
 
 int tty_prepare_flip_string_flags(struct tty_struct *tty, unsigned char **chars, char **flags, size_t size)
@@ -451,10 +543,16 @@ EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);
 
 
 
-/*
+/**
+ *      tty_set_termios_ldisc           -       set ldisc field
+ *      @tty: tty structure
+ *      @num: line discipline number
+ *
  *      This is probably overkill for real world processors but
  *      they are not on hot paths so a little discipline won't do 
  *      any harm.
+ *
+ *      Locking: takes termios_sem
  */
  
 static void tty_set_termios_ldisc(struct tty_struct *tty, int num)
@@ -474,6 +572,19 @@ static DEFINE_SPINLOCK(tty_ldisc_lock);
 static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
 static struct tty_ldisc tty_ldiscs[NR_LDISCS];  /* line disc dispatch table */
 
+/**
+ *      tty_register_ldisc      -       install a line discipline
+ *      @disc: ldisc number
+ *      @new_ldisc: pointer to the ldisc object
+ *
+ *      Installs a new line discipline into the kernel. The discipline
+ *      is set up as unreferenced and then made available to the kernel
+ *      from this point onwards.
+ *
+ *      Locking:
+ *              takes tty_ldisc_lock to guard against ldisc races
+ */
+
 int tty_register_ldisc(int disc, struct tty_ldisc *new_ldisc)
 {
         unsigned long flags;
@@ -493,6 +604,18 @@ int tty_register_ldisc(int disc, struct tty_ldisc *new_ldisc)
 }
 EXPORT_SYMBOL(tty_register_ldisc);
 
+/**
+ *      tty_unregister_ldisc    -       unload a line discipline
+ *      @disc: ldisc number
+ *      @new_ldisc: pointer to the ldisc object
+ *
+ *      Remove a line discipline from the kernel providing it is not
+ *      currently in use.
+ *
+ *      Locking:
+ *              takes tty_ldisc_lock to guard against ldisc races
+ */
+
 int tty_unregister_ldisc(int disc)
 {
         unsigned long flags;
@@ -512,6 +635,19 @@ int tty_unregister_ldisc(int disc)
 }
 EXPORT_SYMBOL(tty_unregister_ldisc);
 
+/**
+ *      tty_ldisc_get           -       take a reference to an ldisc
+ *      @disc: ldisc number
+ *
+ *      Takes a reference to a line discipline. Deals with refcounts and
+ *      module locking counts. Returns NULL if the discipline is not available.
+ *      Returns a pointer to the discipline and bumps the ref count if it is
+ *      available
+ *
+ *      Locking:
+ *              takes tty_ldisc_lock to guard against ldisc races
+ */
+
 struct tty_ldisc *tty_ldisc_get(int disc)
 {
         unsigned long flags;
@@ -540,6 +676,17 @@ struct tty_ldisc *tty_ldisc_get(int disc)
 
 EXPORT_SYMBOL_GPL(tty_ldisc_get);
 
+/**
+ *      tty_ldisc_put           -       drop ldisc reference
+ *      @disc: ldisc number
+ *
+ *      Drop a reference to a line discipline. Manage refcounts and
+ *      module usage counts
+ *
+ *      Locking:
+ *              takes tty_ldisc_lock to guard against ldisc races
+ */
+
 void tty_ldisc_put(int disc)
 {
         struct tty_ldisc *ld;
@@ -557,6 +704,19 @@ void tty_ldisc_put(int disc)
         
 EXPORT_SYMBOL_GPL(tty_ldisc_put);
 
+/**
+ *      tty_ldisc_assign        -       set ldisc on a tty
+ *      @tty: tty to assign
+ *      @ld: line discipline
+ *
+ *      Install an instance of a line discipline into a tty structure. The
+ *      ldisc must have a reference count above zero to ensure it remains/
+ *      The tty instance refcount starts at zero.
+ *
+ *      Locking:
+ *              Caller must hold references
+ */
+
 static void tty_ldisc_assign(struct tty_struct *tty, struct tty_ldisc *ld)
 {
         tty->ldisc = *ld;
@@ -571,6 +731,8 @@ static void tty_ldisc_assign(struct tty_struct *tty, struct tty_ldisc *ld)
  *      the tty ldisc. Return 0 on failure or 1 on success. This is
  *      used to implement both the waiting and non waiting versions
  *      of tty_ldisc_ref
+ *
+ *      Locking: takes tty_ldisc_lock
  */
 
 static int tty_ldisc_try(struct tty_struct *tty)
@@ -602,6 +764,8 @@ static int tty_ldisc_try(struct tty_struct *tty)
  *      must also be careful not to hold other locks that will deadlock
  *      against a discipline change, such as an existing ldisc reference
  *      (which we check for)
+ *
+ *      Locking: call functions take tty_ldisc_lock
  */
  
 struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
@@ -622,6 +786,8 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
  *      Dereference the line discipline for the terminal and take a 
  *      reference to it. If the line discipline is in flux then 
  *      return NULL. Can be called from IRQ and timer functions.
+ *
+ *      Locking: called functions take tty_ldisc_lock
  */
  
 struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
@@ -639,6 +805,8 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref);
  *
  *      Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
  *      be called in IRQ context.
+ *
+ *      Locking: takes tty_ldisc_lock
  */
  
 void tty_ldisc_deref(struct tty_ldisc *ld)
@@ -683,6 +851,9 @@ static void tty_ldisc_enable(struct tty_struct *tty)
  *
  *      Set the discipline of a tty line. Must be called from a process
  *      context.
+ *
+ *      Locking: takes tty_ldisc_lock.
+ *              called functions take termios_sem
  */
  
 static int tty_set_ldisc(struct tty_struct *tty, int ldisc)
@@ -846,9 +1017,17 @@ restart:
         return retval;
 }
 
-/*
- * This routine returns a tty driver structure, given a device number
+/**
+ *      get_tty_driver          -       find device of a tty
+ *      @dev_t: device identifier
+ *      @index: returns the index of the tty
+ *
+ *      This routine returns a tty driver structure, given a device number
+ *      and also passes back the index number.
+ *
+ *      Locking: caller must hold tty_mutex
  */
+
 static struct tty_driver *get_tty_driver(dev_t device, int *index)
 {
         struct tty_driver *p;
@@ -863,11 +1042,17 @@ static struct tty_driver *get_tty_driver(dev_t device, int *index)
         return NULL;
 }
 
-/*
- * If we try to write to, or set the state of, a terminal and we're
- * not in the foreground, send a SIGTTOU.  If the signal is blocked or
- * ignored, go ahead and perform the operation.  (POSIX 7.2)
+/**
+ *      tty_check_change        -       check for POSIX terminal changes
+ *      @tty: tty to check
+ *
+ *      If we try to write to, or set the state of, a terminal and we're
+ *      not in the foreground, send a SIGTTOU.  If the signal is blocked or
+ *      ignored, go ahead and perform the operation.  (POSIX 7.2)
+ *
+ *      Locking: none
  */
+
 int tty_check_change(struct tty_struct * tty)
 {
         if (current->signal->tty != tty)
@@ -1005,10 +1190,27 @@ void tty_ldisc_flush(struct tty_struct *tty)
 
 EXPORT_SYMBOL_GPL(tty_ldisc_flush);
         
-/*
- * This can be called by the "eventd" kernel thread.  That is process synchronous,
- * but doesn't hold any locks, so we need to make sure we have the appropriate
- * locks for what we're doing..
+/**
+ *      do_tty_hangup           -       actual handler for hangup events
+ *      @data: tty device
+ *
+ *      This can be called by the "eventd" kernel thread.  That is process
+ *      synchronous but doesn't hold any locks, so we need to make sure we
+ *      have the appropriate locks for what we're doing.
+ *
+ *      The hangup event clears any pending redirections onto the hung up
+ *      device. It ensures future writes will error and it does the needed
+ *      line discipline hangup and signal delivery. The tty object itself
+ *      remains intact.
+ *
+ *      Locking:
+ *              BKL
+ *              redirect lock for undoing redirection
+ *              file list lock for manipulating list of ttys
+ *              tty_ldisc_lock from called functions
+ *              termios_sem resetting termios data
+ *              tasklist_lock to walk task list for hangup event
+ *
  */
 static void do_tty_hangup(void *data)
 {
@@ -1133,6 +1335,14 @@ static void do_tty_hangup(void *data)
                 fput(f);
 }
 
+/**
+ *      tty_hangup              -       trigger a hangup event
+ *      @tty: tty to hangup
+ *
+ *      A carrier loss (virtual or otherwise) has occurred on this like
+ *      schedule a hangup sequence to run after this event.
+ */
+
 void tty_hangup(struct tty_struct * tty)
 {
 #ifdef TTY_DEBUG_HANGUP
@@ -1145,6 +1355,15 @@ void tty_hangup(struct tty_struct * tty)
 
 EXPORT_SYMBOL(tty_hangup);
 
+/**
+ *      tty_vhangup             -       process vhangup
+ *      @tty: tty to hangup
+ *
+ *      The user has asked via system call for the terminal to be hung up.
+ *      We do this synchronously so that when the syscall returns the process
+ *      is complete. That guarantee is neccessary for security reasons.
+ */
+
 void tty_vhangup(struct tty_struct * tty)
 {
 #ifdef TTY_DEBUG_HANGUP
@@ -1156,6 +1375,14 @@ void tty_vhangup(struct tty_struct * tty)
 }
 EXPORT_SYMBOL(tty_vhangup);
 
+/**
+ *      tty_hung_up_p           -       was tty hung up
+ *      @filp: file pointer of tty
+ *
+ *      Return true if the tty has been subject to a vhangup or a carrier
+ *      loss
+ */
+
 int tty_hung_up_p(struct file * filp)
 {
         return (filp->f_op == &hung_up_tty_fops);
@@ -1163,19 +1390,28 @@ int tty_hung_up_p(struct file * filp)
 
 EXPORT_SYMBOL(tty_hung_up_p);
 
-/*
- * This function is typically called only by the session leader, when
- * it wants to disassociate itself from its controlling tty.
+/**
+ *      disassociate_ctty       -       disconnect controlling tty
+ *      @on_exit: true if exiting so need to "hang up" the session
  *
- * It performs the following functions:
+ *      This function is typically called only by the session leader, when
+ *      it wants to disassociate itself from its controlling tty.
+ *
+ *      It performs the following functions:
  *      (1)  Sends a SIGHUP and SIGCONT to the foreground process group
  *      (2)  Clears the tty from being controlling the session
  *      (3)  Clears the controlling tty for all processes in the
  *              session group.
  *
- * The argument on_exit is set to 1 if called when a process is
- * exiting; it is 0 if called by the ioctl TIOCNOTTY.
+ *      The argument on_exit is set to 1 if called when a process is
+ *      exiting; it is 0 if called by the ioctl TIOCNOTTY.
+ *
+ *      Locking: tty_mutex is taken to protect current->signal->tty
+ *              BKL is taken for hysterical raisins
+ *              Tasklist lock is taken (under tty_mutex) to walk process
+ *              lists for the session.
  */
+
 void disassociate_ctty(int on_exit)
 {
         struct tty_struct *tty;
@@ -1222,6 +1458,25 @@ void disassociate_ctty(int on_exit)
         unlock_kernel();
 }
 
+
+/**
+ *      stop_tty        -       propogate flow control
+ *      @tty: tty to stop
+ *
+ *      Perform flow control to the driver. For PTY/TTY pairs we
+ *      must also propogate the TIOCKPKT status. May be called
+ *      on an already stopped device and will not re-call the driver
+ *      method.
+ *
+ *      This functionality is used by both the line disciplines for
+ *      halting incoming flow and by the driver. It may therefore be
+ *      called from any context, may be under the tty atomic_write_lock
+ *      but not always.
+ *
+ *      Locking:
+ *              Broken. Relies on BKL which is unsafe here.
+ */
+
 void stop_tty(struct tty_struct *tty)
 {
         if (tty->stopped)
@@ -1238,6 +1493,19 @@ void stop_tty(struct tty_struct *tty)
 
 EXPORT_SYMBOL(stop_tty);
 
+/**
+ *      start_tty       -       propogate flow control
+ *      @tty: tty to start
+ *
+ *      Start a tty that has been stopped if at all possible. Perform
+ *      any neccessary wakeups and propogate the TIOCPKT status. If this
+ *      is the tty was previous stopped and is being started then the
+ *      driver start method is invoked and the line discipline woken.
+ *
+ *      Locking:
+ *              Broken. Relies on BKL which is unsafe here.
+ */
+
 void start_tty(struct tty_struct *tty)
 {
         if (!tty->stopped || tty->flow_stopped)
@@ -1258,6 +1526,23 @@ void start_tty(struct tty_struct *tty)
 
 EXPORT_SYMBOL(start_tty);
 
+/**
+ *      tty_read        -       read method for tty device files
+ *      @file: pointer to tty file
+ *      @buf: user buffer
+ *      @count: size of user buffer
+ *      @ppos: unused
+ *
+ *      Perform the read system call function on this terminal device. Checks
+ *      for hung up devices before calling the line discipline method.
+ *
+ *      Locking:
+ *              Locks the line discipline internally while needed
+ *              For historical reasons the line discipline read method is
+ *      invoked under the BKL. This will go away in time so do not rely on it
+ *      in new code. Multiple read calls may be outstanding in parallel.
+ */
+
 static ssize_t tty_read(struct file * file, char __user * buf, size_t count, 
                         loff_t *ppos)
 {
@@ -1302,6 +1587,7 @@ static inline ssize_t do_tty_write(
         ssize_t ret = 0, written = 0;
         unsigned int chunk;
         
+        /* FIXME: O_NDELAY ... */
         if (mutex_lock_interruptible(&tty->atomic_write_lock)) {
                 return -ERESTARTSYS;
         }
@@ -1318,6 +1604,9 @@ static inline ssize_t do_tty_write(
          * layer has problems with bigger chunks. It will
          * claim to be able to handle more characters than
          * it actually does.
+         *
+         * FIXME: This can probably go away now except that 64K chunks
+         * are too likely to fail unless switched to vmalloc...
          */
         chunk = 2048;
         if (test_bit(TTY_NO_WRITE_SPLIT, &tty->flags))
@@ -1375,6 +1664,24 @@ static inline ssize_t do_tty_write(
 }
 
 
+/**
+ *      tty_write               -       write method for tty device file
+ *      @file: tty file pointer
+ *      @buf: user data to write
+ *      @count: bytes to write
+ *      @ppos: unused
+ *
+ *      Write data to a tty device via the line discipline.
+ *
+ *      Locking:
+ *              Locks the line discipline as required
+ *              Writes to the tty driver are serialized by the atomic_write_lock
+ *      and are then processed in chunks to the device. The line discipline
+ *      write method will not be involked in parallel for each device
+ *              The line discipline write method is called under the big
+ *      kernel lock for historical reasons. New code should not rely on this.
+ */
+
 static ssize_t tty_write(struct file * file, const char __user * buf, size_t count,
                          loff_t *ppos)
 {
@@ -1422,7 +1729,18 @@ ssize_t redirected_tty_write(struct file * file, const char __user * buf, size_t
 
 static char ptychar[] = "pqrstuvwxyzabcde";
 
-static inline void pty_line_name(struct tty_driver *driver, int index, char *p)
+/**
+ *      pty_line_name   -       generate name for a pty
+ *      @driver: the tty driver in use
+ *      @index: the minor number
+ *      @p: output buffer of at least 6 bytes
+ *
+ *      Generate a name from a driver reference and write it to the output
+ *      buffer.
+ *
+ *      Locking: None
+ */
+static void pty_line_name(struct tty_driver *driver, int index, char *p)
 {
         int i = index + driver->name_base;
         /* ->name is initialized to "ttyp", but "tty" is expected */
@@ -1431,24 +1749,53 @@ static inline void pty_line_name(struct tty_driver *driver, int index, char *p)
                         ptychar[i >> 4 & 0xf], i & 0xf);
 }
 
-static inline void tty_line_name(struct tty_driver *driver, int index, char *p)
+/**
+ *      pty_line_name   -       generate name for a tty
+ *      @driver: the tty driver in use
+ *      @index: the minor number
+ *      @p: output buffer of at least 7 bytes
+ *
+ *      Generate a name from a driver reference and write it to the output
+ *      buffer.
+ *
+ *      Locking: None
+ */
+static void tty_line_name(struct tty_driver *driver, int index, char *p)
 {
         sprintf(p, "%s%d", driver->name, index + driver->name_base);
 }
 
-/*
+/**
+ *      init_dev                -       initialise a tty device
+ *      @driver: tty driver we are opening a device on
+ *      @idx: device index
+ *      @tty: returned tty structure
+ *
+ *      Prepare a tty device. This may not be a "new" clean device but
+ *      could also be an active device. The pty drivers require special
+ *      handling because of this.
+ *
+ *      Locking:
+ *              The function is called under the tty_mutex, which
+ *      protects us from the tty struct or driver itself going away.
+ *
+ *      On exit the tty device has the line discipline attached and
+ *      a reference count of 1. If a pair was created for pty/tty use
+ *      and the other was a pty master then it too has a reference count of 1.
+ *
  * WSH 06/09/97: Rewritten to remove races and properly clean up after a
  * failed open.  The new code protects the open with a mutex, so it's
  * really quite straightforward.  The mutex locking can probably be
  * relaxed for the (most common) case of reopening a tty.
  */
+
 static int init_dev(struct tty_driver *driver, int idx,
         struct tty_struct **ret_tty)
 {
         struct tty_struct *tty, *o_tty;
         struct termios *tp, **tp_loc, *o_tp, **o_tp_loc;
         struct termios *ltp, **ltp_loc, *o_ltp, **o_ltp_loc;
-        int retval=0;
+        int retval = 0;
 
         /* check whether we're reopening an existing tty */
         if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
@@ -1662,10 +2009,20 @@ release_mem_out:
         goto end_init;
 }
 
-/*
- * Releases memory associated with a tty structure, and clears out the
- * driver table slots.
+/**
+ *      release_mem             -       release tty structure memory
+ *
+ *      Releases memory associated with a tty structure, and clears out the
+ *      driver table slots. This function is called when a device is no longer
+ *      in use. It also gets called when setup of a device fails.
+ *
+ *      Locking:
+ *              tty_mutex - sometimes only
+ *              takes the file list lock internally when working on the list
+ *      of ttys that the driver keeps.
+ *              FIXME: should we require tty_mutex is held here ??
  */
+
 static void release_mem(struct tty_struct *tty, int idx)
 {
         struct tty_struct *o_tty;
@@ -2006,18 +2363,27 @@ static void release_dev(struct file * filp)
 
 }
 
-/*
- * tty_open and tty_release keep up the tty count that contains the
- * number of opens done on a tty. We cannot use the inode-count, as
- * different inodes might point to the same tty.
+/**
+ *      tty_open                -       open a tty device
+ *      @inode: inode of device file
+ *      @filp: file pointer to tty
  *
- * Open-counting is needed for pty masters, as well as for keeping
- * track of serial lines: DTR is dropped when the last close happens.
- * (This is not done solely through tty->count, now.  - Ted 1/27/92)
+ *      tty_open and tty_release keep up the tty count that contains the
+ *      number of opens done on a tty. We cannot use the inode-count, as
+ *      different inodes might point to the same tty.
  *
- * The termios state of a pty is reset on first open so that
- * settings don't persist across reuse.
+ *      Open-counting is needed for pty masters, as well as for keeping
+ *      track of serial lines: DTR is dropped when the last close happens.
+ *      (This is not done solely through tty->count, now.  - Ted 1/27/92)
+ *
+ *      The termios state of a pty is reset on first open so that
+ *      settings don't persist across reuse.
+ *
+ *      Locking: tty_mutex protects current->signal->tty, get_tty_driver and
+ *              init_dev work. tty->count should protect the rest.
+ *              task_lock is held to update task details for sessions
  */
+
 static int tty_open(struct inode * inode, struct file * filp)
 {
         struct tty_struct *tty;
@@ -2132,6 +2498,18 @@ got_driver:
 }
 
 #ifdef CONFIG_UNIX98_PTYS
+/**
+ *      ptmx_open               -       open a unix 98 pty master
+ *      @inode: inode of device file
+ *      @filp: file pointer to tty
+ *
+ *      Allocate a unix98 pty master device from the ptmx driver.
+ *
+ *      Locking: tty_mutex protects theinit_dev work. tty->count should
+                protect the rest.
+ *              allocated_ptys_lock handles the list of free pty numbers
+ */
+
 static int ptmx_open(struct inode * inode, struct file * filp)
 {
         struct tty_struct *tty;
@@ -2191,6 +2569,18 @@ out:
 }
 #endif
 
+/**
+ *      tty_release             -       vfs callback for close
+ *      @inode: inode of tty
+ *      @filp: file pointer for handle to tty
+ *
+ *      Called the last time each file handle is closed that references
+ *      this tty. There may however be several such references.
+ *
+ *      Locking:
+ *              Takes bkl. See release_dev
+ */
+
 static int tty_release(struct inode * inode, struct file * filp)
 {
         lock_kernel();
@@ -2199,7 +2589,18 @@ static int tty_release(struct inode * inode, struct file * filp)
         return 0;
 }
 
-/* No kernel lock held - fine */
+/**
+ *      tty_poll        -       check tty status
+ *      @filp: file being polled
+ *      @wait: poll wait structures to update
+ *
+ *      Call the line discipline polling method to obtain the poll
+ *      status of the device.
+ *
+ *      Locking: locks called line discipline but ldisc poll method
+ *      may be re-entered freely by other callers.
+ */
+
 static unsigned int tty_poll(struct file * filp, poll_table * wait)
 {
         struct tty_struct * tty;
@@ -2243,6 +2644,21 @@ static int tty_fasync(int fd, struct file * filp, int on)
         return 0;
 }
 
+/**
+ *      tiocsti                 -       fake input character
+ *      @tty: tty to fake input into
+ *      @p: pointer to character
+ *
+ *      Fake input to a tty device. Does the neccessary locking and
+ *      input management.
+ *
+ *      FIXME: does not honour flow control ??
+ *
+ *      Locking:
+ *              Called functions take tty_ldisc_lock
+ *              current->signal->tty check is safe without locks
+ */
+
 static int tiocsti(struct tty_struct *tty, char __user *p)
 {
         char ch, mbz = 0;
@@ -2258,6 +2674,18 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
         return 0;
 }
 
+/**
+ *      tiocgwinsz              -       implement window query ioctl
+ *      @tty; tty
+ *      @arg: user buffer for result
+ *
+ *      Copies the kernel idea of the window size into the user buffer. No
+ *      locking is done.
+ *
+ *      FIXME: Returning random values racing a window size set is wrong
+ *      should lock here against that
+ */
+
 static int tiocgwinsz(struct tty_struct *tty, struct winsize __user * arg)
 {
         if (copy_to_user(arg, &tty->winsize, sizeof(*arg)))
@@ -2265,6 +2693,24 @@ static int tiocgwinsz(struct tty_struct *tty, struct winsize __user * arg)
         return 0;
 }
 
+/**
+ *      tiocswinsz              -       implement window size set ioctl
+ *      @tty; tty
+ *      @arg: user buffer for result
+ *
+ *      Copies the user idea of the window size to the kernel. Traditionally
+ *      this is just advisory information but for the Linux console it
+ *      actually has driver level meaning and triggers a VC resize.
+ *
+ *      Locking:
+ *              The console_sem is used to ensure we do not try and resize
+ *      the console twice at once.
+ *      FIXME: Two racing size sets may leave the console and kernel
+ *              parameters disagreeing. Is this exploitable ?
+ *      FIXME: Random values racing a window size get is wrong
+ *      should lock here against that
+ */
+
 static int tiocswinsz(struct tty_struct *tty, struct tty_struct *real_tty,
         struct winsize __user * arg)
 {
@@ -2294,6 +2740,15 @@ static int tiocswinsz(struct tty_struct *tty, struct tty_struct *real_tty,
         return 0;
 }
 
+/**
+ *      tioccons        -       allow admin to move logical console
+ *      @file: the file to become console
+ *
+ *      Allow the adminstrator to move the redirected console device
+ *
+ *      Locking: uses redirect_lock to guard the redirect information
+ */
+
 static int tioccons(struct file *file)
 {
         if (!capable(CAP_SYS_ADMIN))
@@ -2319,6 +2774,17 @@ static int tioccons(struct file *file)
         return 0;
 }
 
+/**
+ *      fionbio         -       non blocking ioctl
+ *      @file: file to set blocking value
+ *      @p: user parameter
+ *
+ *      Historical tty interfaces had a blocking control ioctl before
+ *      the generic functionality existed. This piece of history is preserved
+ *      in the expected tty API of posix OS's.
+ *
+ *      Locking: none, the open fle handle ensures it won't go away.
+ */
 
 static int fionbio(struct file *file, int __user *p)
 {
@@ -2334,6 +2800,23 @@ static int fionbio(struct file *file, int __user *p)
         return 0;
 }
 
+/**
+ *      tiocsctty       -       set controlling tty
+ *      @tty: tty structure
+ *      @arg: user argument
+ *
+ *      This ioctl is used to manage job control. It permits a session
+ *      leader to set this tty as the controlling tty for the session.
+ *
+ *      Locking:
+ *              Takes tasklist lock internally to walk sessions
+ *              Takes task_lock() when updating signal->tty
+ *
+ *      FIXME: tty_mutex is needed to protect signal->tty references.
+ *      FIXME: why task_lock on the signal->tty reference ??
+ *
+ */
+
 static int tiocsctty(struct tty_struct *tty, int arg)
 {
         struct task_struct *p;
@@ -2374,6 +2857,18 @@ static int tiocsctty(struct tty_struct *tty, int arg)
         return 0;
 }
 
+/**
+ *      tiocgpgrp               -       get process group
+ *      @tty: tty passed by user
+ *      @real_tty: tty side of the tty pased by the user if a pty else the tty
+ *      @p: returned pid
+ *
+ *      Obtain the process group of the tty. If there is no process group
+ *      return an error.
+ *
+ *      Locking: none. Reference to ->signal->tty is safe.
+ */
+
 static int tiocgpgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
 {
         /*
@@ -2385,6 +2880,20 @@ static int tiocgpgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
         return put_user(real_tty->pgrp, p);
 }
 
+/**
+ *      tiocspgrp               -       attempt to set process group
+ *      @tty: tty passed by user
+ *      @real_tty: tty side device matching tty passed by user
+ *      @p: pid pointer
+ *
+ *      Set the process group of the tty to the session passed. Only
+ *      permitted where the tty session is our session.
+ *
+ *      Locking: None
+ *
+ *      FIXME: current->signal->tty referencing is unsafe.
+ */
+
 static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
 {
         pid_t pgrp;
@@ -2408,6 +2917,18 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
         return 0;
 }
 
+/**
+ *      tiocgsid                -       get session id
+ *      @tty: tty passed by user
+ *      @real_tty: tty side of the tty pased by the user if a pty else the tty
+ *      @p: pointer to returned session id
+ *
+ *      Obtain the session id of the tty. If there is no session
+ *      return an error.
+ *
+ *      Locking: none. Reference to ->signal->tty is safe.
+ */
+
 static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
 {
         /*
@@ -2421,6 +2942,16 @@ static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t _
         return put_user(real_tty->session, p);
 }
 
+/**
+ *      tiocsetd        -       set line discipline
+ *      @tty: tty device
+ *      @p: pointer to user data
+ *
+ *      Set the line discipline according to user request.
+ *
+ *      Locking: see tty_set_ldisc, this function is just a helper
+ */
+
 static int tiocsetd(struct tty_struct *tty, int __user *p)
 {
         int ldisc;
@@ -2430,6 +2961,21 @@ static int tiocsetd(struct tty_struct *tty, int __user *p)
         return tty_set_ldisc(tty, ldisc);
 }
 
+/**
+ *      send_break      -       performed time break
+ *      @tty: device to break on
+ *      @duration: timeout in mS
+ *
+ *      Perform a timed break on hardware that lacks its own driver level
+ *      timed break functionality.
+ *
+ *      Locking:
+ *              None
+ *
+ *      FIXME:
+ *              What if two overlap
+ */
+
 static int send_break(struct tty_struct *tty, unsigned int duration)
 {
         tty->driver->break_ctl(tty, -1);
@@ -2442,8 +2988,19 @@ static int send_break(struct tty_struct *tty, unsigned int duration)
         return 0;
 }
 
-static int
-tty_tiocmget(struct tty_struct *tty, struct file *file, int __user *p)
+/**
+ *      tiocmget                -       get modem status
+ *      @tty: tty device
+ *      @file: user file pointer
+ *      @p: pointer to result
+ *
+ *      Obtain the modem status bits from the tty driver if the feature
+ *      is supported. Return -EINVAL if it is not available.
+ *
+ *      Locking: none (up to the driver)
+ */
+
+static int tty_tiocmget(struct tty_struct *tty, struct file *file, int __user *p)
 {
         int retval = -EINVAL;
 
@@ -2456,8 +3013,20 @@ tty_tiocmget(struct tty_struct *tty, struct file *file, int __user *p)
         return retval;
 }
 
-static int
-tty_tiocmset(struct tty_struct *tty, struct file *file, unsigned int cmd,
+/**
+ *      tiocmset                -       set modem status
+ *      @tty: tty device
+ *      @file: user file pointer
+ *      @cmd: command - clear bits, set bits or set all
+ *      @p: pointer to desired bits
+ *
+ *      Set the modem status bits from the tty driver if the feature
+ *      is supported. Return -EINVAL if it is not available.
+ *
+ *      Locking: none (up to the driver)
+ */
+
+static int tty_tiocmset(struct tty_struct *tty, struct file *file, unsigned int cmd,
              unsigned __user *p)
 {
         int retval = -EINVAL;
@@ -2573,6 +3142,7 @@ int tty_ioctl(struct inode * inode, struct file * file,
                         clear_bit(TTY_EXCLUSIVE, &tty->flags);
                         return 0;
                 case TIOCNOTTY:
+                        /* FIXME: taks lock or tty_mutex ? */
                         if (current->signal->tty != tty)
                                 return -ENOTTY;
                         if (current->signal->leader)
@@ -2753,9 +3323,16 @@ void do_SAK(struct tty_struct *tty)
 
 EXPORT_SYMBOL(do_SAK);
 
-/*
- * This routine is called out of the software interrupt to flush data
- * from the buffer chain to the line discipline.
+/**
+ *      flush_to_ldisc
+ *      @private_: tty structure passed from work queue.
+ *
+ *      This routine is called out of the software interrupt to flush data
+ *      from the buffer chain to the line discipline.
+ *
+ *      Locking: holds tty->buf.lock to guard buffer list. Drops the lock
+ *      while invoking the line discipline receive_buf method. The
+ *      receive_buf method is single threaded for each tty instance.
  */
  
 static void flush_to_ldisc(void *private_)
@@ -2831,6 +3408,8 @@ static int n_baud_table = ARRAY_SIZE(baud_table);
  *      Convert termios baud rate data into a speed. This should be called
  *      with the termios lock held if this termios is a terminal termios
  *      structure. May change the termios data.
+ *
+ *      Locking: none
  */
  
 int tty_termios_baud_rate(struct termios *termios)
@@ -2859,6 +3438,8 @@ EXPORT_SYMBOL(tty_termios_baud_rate);
  *      Returns the baud rate as an integer for this terminal. The
  *      termios lock must be held by the caller and the terminal bit
  *      flags may be updated.
+ *
+ *      Locking: none
  */
  
 int tty_get_baud_rate(struct tty_struct *tty)
@@ -2888,6 +3469,8 @@ EXPORT_SYMBOL(tty_get_baud_rate);
  *
  *      In the event of the queue being busy for flipping the work will be
  *      held off and retried later.
+ *
+ *      Locking: tty buffer lock. Driver locks in low latency mode.
  */
 
 void tty_flip_buffer_push(struct tty_struct *tty)
@@ -2907,9 +3490,16 @@ void tty_flip_buffer_push(struct tty_struct *tty)
 EXPORT_SYMBOL(tty_flip_buffer_push);
 
 
-/*
- * This subroutine initializes a tty structure.
+/**
+ *      initialize_tty_struct
+ *      @tty: tty to initialize
+ *
+ *      This subroutine initializes a tty structure that has been newly
+ *      allocated.
+ *
+ *      Locking: none - tty in question must not be exposed at this point
  */
+
 static void initialize_tty_struct(struct tty_struct *tty)
 {
         memset(tty, 0, sizeof(struct tty_struct));
@@ -2935,6 +3525,7 @@ static void initialize_tty_struct(struct tty_struct *tty)
 /*
  * The default put_char routine if the driver did not define one.
  */
+
 static void tty_default_put_char(struct tty_struct *tty, unsigned char ch)
 {
         tty->driver->write(tty, &ch, 1);
@@ -2943,19 +3534,23 @@ static void tty_default_put_char(struct tty_struct *tty, unsigned char ch)
 static struct class *tty_class;
 
 /**
- * tty_register_device - register a tty device
- * @driver: the tty driver that describes the tty device
- * @index: the index in the tty driver for this tty device
- * @device: a struct device that is associated with this tty device.
- *      This field is optional, if there is no known struct device for this
- *      tty device it can be set to NULL safely.
+ *      tty_register_device - register a tty device
+ *      @driver: the tty driver that describes the tty device
+ *      @index: the index in the tty driver for this tty device
+ *      @device: a struct device that is associated with this tty device.
+ *              This field is optional, if there is no known struct device
+ *              for this tty device it can be set to NULL safely.
  *
- * Returns a pointer to the class device (or ERR_PTR(-EFOO) on error).
+ *      Returns a pointer to the class device (or ERR_PTR(-EFOO) on error).
  *
- * This call is required to be made to register an individual tty device if
- * the tty driver's flags have the TTY_DRIVER_DYNAMIC_DEV bit set.  If that
- * bit is not set, this function should not be called by a tty driver.
+ *      This call is required to be made to register an individual tty device
+ *      if the tty driver's flags have the TTY_DRIVER_DYNAMIC_DEV bit set.  If
+ *      that bit is not set, this function should not be called by a tty
+ *      driver.
+ *
+ *      Locking: ??
  */
+
 struct class_device *tty_register_device(struct tty_driver *driver,
                                          unsigned index, struct device *device)
 {
@@ -2977,13 +3572,16 @@ struct class_device *tty_register_device(struct tty_driver *driver,
 }
 
 /**
- * tty_unregister_device - unregister a tty device
- * @driver: the tty driver that describes the tty device
- * @index: the index in the tty driver for this tty device
+ *      tty_unregister_device - unregister a tty device
+ *      @driver: the tty driver that describes the tty device
+ *      @index: the index in the tty driver for this tty device
  *
- * If a tty device is registered with a call to tty_register_device() then
- * this function must be made when the tty device is gone.
+ *      If a tty device is registered with a call to tty_register_device() then
+ *      this function must be called when the tty device is gone.
+ *
+ *      Locking: ??
  */
+
 void tty_unregister_device(struct tty_driver *driver, unsigned index)
 {
         class_device_destroy(tty_class, MKDEV(driver->major, driver->minor_start) + index);