In the original MPSAFE TTY code, I changed the behaviour by returning
EBUSY. I thought this made more sense, because it's basically a race to
see who gets the TTY first.
It turns out this is not a good change, because it also causes EBUSY to
be returned when another process is closing the TTY. This can happen
during startup, when /etc/rc (or one of its children) is still busy
draining its data and /sbin/init is attempting to open the TTY to spawn
a getty.
Reported by: bz
Tested by: bz
fget_unlocked().
- Save old file descriptor tables created on expansion until
the entire descriptor table is freed so that pointers may be
followed without regard for expanders.
- Mark the file zone as NOFREE so we may attempt to reference
potentially freed files.
- Convert several fget_locked() users to fget_unlocked(). This
requires us to manage reference counts explicitly but reduces
locking overhead in the common case.
It turns out my handling of SIGTTOU and SIGTTIN didn't entirely comply
to the standards. It is true that in the SIGTTOU case we should not
return EIO when the signal is ignored/blocked, but in the SIGTTIN case
we must.
See also: POSIX issue 7 section 11.1.4
It's better to just use internal language constructs, because it is
likely the compiler has a better opinion on whether to perform inlining,
which is very likely to happen to struct winsize.
Submitted by: Christoph Mallon <christoph mallon gmx de>
Just like the old TTY layer, the current MPSAFE TTY layer does not make
any attempt to serialize calls of write(). Data is copied into the
kernel in 256 (TTY_STACKBUF) byte chunks. If a write() call occurs at
the same time, the data may interleave. This is especially likely when
the TTY starts blocking, because the output queue reaches the high
watermark.
I've implemented this by adding a new flag, TTY_BUSY_OUT, which is used
to mark a TTY as having a thread stuck in write(). Because I don't want
non-blocking processes to be possibly blocked by a sleeping thread, I'm
still allowing it to bypass the protection. According to this message,
the Linux kernel returns EAGAIN in such cases, but I think that's a
little too restrictive:
http://kerneltrap.org/index.php?q=mailarchive/linux-kernel/2007/5/2/85418/thread
PR: kern/118287
When we leave the console TTY constantly open, we never reset the
termios attributes. This causes output processing, echoing, etc. not to
be reset to the proper values when going into single user mode after the
system has booted. It also causes nl-to-crnl-conversion not to take
place during shutdown, which causes a `staircase effect'.
This patch adds a new TTY flag, TF_OPENED_CONS, which is set when the
TTY is opened through /dev/console. Because the flags are only used by
the kernel and the pstat(8) utility, I've decided to renumber the TTY
flags. This shouldn't be an issue, because the TTY layer is not yet part
of a stable release.
Reported by: Mark Atkinson <atkin901 yahoo com>
Tested by: sepotvin
The TTY buffers used the standard <sys/queue.h> lists. Unfortunately
they have a big shortcoming. If you want to have a double linked list,
but no tail pointer, it's still not possible to obtain the previous
element in the list. Inside the buffers we don't need them. This is why
I switched to custom linked list macros. The macros will also keep track
of the amount of items in the list. Because it doesn't use a sentinel,
we can just initialize the queues with zero.
In its simplest form (the output queue), we will only keep two
references to blocks in the queue, namely the head of the list and the
last block in use. All free blocks are stored behind the last block in
use.
I noticed there was a very subtle bug in the previous code: in a very
uncommon corner case, it would uma_zfree() a block in the queue before
calling memcpy() to extract the data from the block.
During startup some of the syscons TTY's are used to set attributes like
the screensaver and mouse options. These actions cause /dev/console to
be rendered unusable.
Fix the issue by leaving the TTY opened when it is used as the console
device.
Reported by: imp
Right now the wchan strings "ttyinp" and "ttybgw" only differ one
character from the strings we used prior to MPSAFE TTY. Just rename them
back to their pre-MPSAFE TTY counterparts.
Also rename "ttylck" to "ttymtx", which should make it more clear that a
process is blocked on the TTY mutex, not some other form of locking.
On RELENG_6 (and probably RELENG_7) we see our syscons windows and
pseudo-terminals have the following buffer sizes:
| LINE RAW CAN OUT IHIWT ILOWT OHWT LWT COL STATE SESS PGID DISC
| ttyv0 0 0 0 7680 6720 2052 256 7 OCcl 1146 1146 term
| ttyp0 0 0 0 7680 6720 1296 256 0 OCc 82033 82033 term
These buffer sizes make no sense, because we often have much more output
than input, but I guess having higher input buffer sizes improves
guarantees of the system.
On MPSAFE TTY I just sent both the input and output buffer sizes to 7
KB, which is pretty big on a standard FreeBSD install with 8 syscons
windows and some PTY's. Reduce the baud rate to 9600 baud, which means
we now have the following buffer sizes:
| LINE INQ CAN LIN LOW OUTQ USE LOW COL SESS PGID STATE
| ttyv0 1920 0 0 192 1984 0 199 7 2401 2401 Oil
| pts/0 1920 0 0 192 1984 0 199 5631 1305 2526 Oi
This is a lot smaller, but for pseudo-devices this should be good
enough. You need to do a lot of punching to fill up a 7.5 KB input
buffer. If it turns out things don't work out this way, we'll just
switch to 19200 baud.
We often run into these very high column numbers when we run curses
applications, because they don't print any newlines. This messes up the
table output of `pstat -t'. If these numbers get really high, they
aren't of any use to the reader anyway. Convert them to `99999' when
they run out of bounds.
One of the pieces of code that I had left alone during the development
of the MPSAFE TTY layer, was tty_cons.c. This file actually has two
different functions:
- It contains low-level console input/output routines (cnputc(), etc).
- It creates /dev/console and wraps all its cdevsw calls to the
appropriate TTY.
This commit reimplements the second set of functions by moving it
directly into the TTY layer. /dev/console is now a character device node
that's basically a regular TTY, but does a lookup of `si_drv1' each time
you open it. d_write has also been changed to call log_console().
d_close() is not present, because we must make sure we don't revoke the
TTY after writing a log message to it.
Even though I'm not convinced this is in line with the future directions
of our console code, it is a good move for now. It removes recursive
locking from the top half of the TTY layer. The previous implementation
called into the TTY layer with Giant held.
I'm renaming tty_cons.c to kern_cons.c now. The code hardly contains any
TTY related bits, so we'd better give it a less misleading name.
Tested by: Andrzej Tobola <ato iem pw edu pl>,
Carlos A.M. dos Santos <unixmania gmail com>,
Eygene Ryabinkin <rea-fbsd codelabs ru>
- Change the ddb(4) commands to be more useful (by thompsa@):
- `show ttys' is now called `show all ttys'. This command will now
also display the address where the TTY data structure resides.
- Add `show tty <addr>', which dumps the TTY in a readable form.
- Place an upper bound on the TTY buffer sizes. Some drivers do not want
to care about baud rates. Protect these drivers by preventing the TTY
buffers from getting enormous. Right now we'll just clamp it to 64K,
which is pretty high, taking into account that these buffers are only
used by the built-in discipline.
- Only call ttydev_leave() when needed. Back in April/May the TTY
reference counting mechanism was a little different, which required us
to call ttydev_leave() each time we finished a cdev operation.
Nowadays we only need to call ttydev_leave() when we really mark it as
being closed.
- Improve return codes of read() and write() on TTY device nodes.
- Make sure we really wake up all blocked threads when the driver calls
tty_rel_gone(). There were some possible code paths where we didn't
properly wake up any readers/writers.
- Add extra assertions to prevent sleeping on a TTY that has been
abandoned by the driver.
- Use ttydev_cdevsw as a more reliable method to figure out whether a
device node is a real TTY device node.
Obtained from: //depot/projects/mpsafetty/...
Reviewed by: thompsa
I've had some reports in the past that opening an already opened TTY
through, for example, /dev/tty can fail with random error codes. Looking
at ttydev_open(), I can see there is a way `error' is returned without
initialising it. Even though I haven't had any confirmation this fixes
the bug, I'll fix it anyway.
Reported by: Andrzej Tobola <ato iem pw edu pl>
Yesterday I got two reports of potential crashes, related to TTY
deallocation during device closure. When a thread is in TF_OPENCLOSE,
draining its output upon closure, we should not allow calls to
tty_rel_free() to happen at the same time. This could cause the TTY to
be torn down twice.
PR: kern/127561
Reported by: KOIE Hidetaka <koie suri co jp>
Discussed with: thompsa
One of the features that prevented us from fixing some of the TTY
consumers to work once again, was an interface that allowed consumers to
do the following:
- `Sniff' incoming data, which is used by the snp(4) driver.
- Take direct control of the input and output paths of a TTY, which is
used by ng_tty(4), ppp(4), sl(4), etc.
There's no practical advantage in committing a hooks layer without
having any consumers. In P4 there is a preliminary port of snp(4) and
thompsa@ is busy porting ng_tty(4) to this interface. I already want to
have it in the tree, because this may stimulate others to work on the
remaining modules.
Discussed with: thompsa
Obtained from: //depot/projects/mpsafetty/...
Unlike tty_rel_gone() and tty_rel_sess(), the tty_rel_pgrp() routine
does not unlock the TTY. I once had the idea to make the code call
tty_rel_pgrp() and tty_rel_sess(), picking up the TTY lock once. This
turned out a little harder than I expected, so this is how it works now.
It's a lot easier if we just let tty_rel_pgrp() unlock the TTY, because
the other routines do this anyway.
As discussed with Robert on IRC, checking the permissions on
/dev/console to see if we can call TIOCCONS could be unreliable. When we
run a chroot() without a devfs instance mounted inside, it won't
actually check the permissions on the device node inside the devfs
instance.
Using the already existing PRIV_TTY_CONSOLE for this seems like a better
idea.
Approved by: rwatson
As reported by several users on the mailing lists, applications like
screen(1) fail to properly handle ^S and ^Q characters. This was because
MPSAFE TTY didn't implement packet mode (TIOCPKT) yet. Add basic packet
mode support to make these applications work again.
Obtained from: //depot/projects/mpsafetty/...
- In the current design, when a TTY decreases its baud rate, it tries to
shrink the queues. This may not always be possible, because it will
not free any blocks that are still filled with data.
Change the TTY queues to store a `quota' value as well, which means it
will not free any blocks when changing the baud rate, but when placing
blocks back into the queue. When the amount of blocks exceeds the
quota, they get freed.
It also fixes some edge cases, where TIOCSETA during read()/
write()-calls could actually make the queue a tiny bit bigger than in
normal cases.
- Don't leak blocks of memory when calling TIOCSETA when the device
driver abandons the TTY while allocating memory.
- Create ttyoutq_init() and ttyinq_init() to initialize the queues,
instead of initializing them by hand. The new TTY snoop driver also
creates an outq, so it's good to have a proper interface to do this.
Obtained from: //depot/projects/mpsafetty/...
For some reason a return-statement crept into this code, where it
shouldn't belong. This means we didn't properly unlock the TTY before
returning to userspace.
Submitted by: Tor Egge <tor egge cvsup no freebsd org>
- According to POSIX, tcsetattr() must not fail when any of the bits in
the structure are unsupported, but it must leave the unsupported flags
alone.
- The CIGNORE flag (set by TCSASOFT, extension) was not cleared from
c_cflag, which means using it would cause it to be applied during its
entire lifespan. Eventually make sure we clear the flag.
I don't really like CIGNORE, but I think we must keep it alive right
now. With our new TTY layer, we don't actually need this mechanism,
because if you leave c_cflag, c_ispeed and c_ospeed alone, we won't make
a call into the device driver anyway.
Reported by: naddy
Tested by: naddy
It turned out we transmitted VSTART after each successful read on a TTY
when software flow control was turned on. This was because of a very
evil bug where we tested the TF_HIWAT_IN flag the other way around.
Reported by: Christian Weisgerber <naddy mips inka de>
The last half year I've been working on a replacement TTY layer for the
FreeBSD kernel. The new TTY layer was designed to improve the following:
- Improved driver model:
The old TTY layer has a driver model that is not abstract enough to
make it friendly to use. A good example is the output path, where the
device drivers directly access the output buffers. This means that an
in-kernel PPP implementation must always convert network buffers into
TTY buffers.
If a PPP implementation would be built on top of the new TTY layer
(still needs a hooks layer, though), it would allow the PPP
implementation to directly hand the data to the TTY driver.
- Improved hotplugging:
With the old TTY layer, it isn't entirely safe to destroy TTY's from
the system. This implementation has a two-step destructing design,
where the driver first abandons the TTY. After all threads have left
the TTY, the TTY layer calls a routine in the driver, which can be
used to free resources (unit numbers, etc).
The pts(4) driver also implements this feature, which means
posix_openpt() will now return PTY's that are created on the fly.
- Improved performance:
One of the major improvements is the per-TTY mutex, which is expected
to improve scalability when compared to the old Giant locking.
Another change is the unbuffered copying to userspace, which is both
used on TTY device nodes and PTY masters.
Upgrading should be quite straightforward. Unlike previous versions,
existing kernel configuration files do not need to be changed, except
when they reference device drivers that are listed in UPDATING.
Obtained from: //depot/projects/mpsafetty/...
Approved by: philip (ex-mentor)
Discussed: on the lists, at BSDCan, at the DevSummit
Sponsored by: Snow B.V., the Netherlands
dcons(4) fixed by: kan
The ttyinfo() routine generates the fancy output when pressing ^T. Right
now it is stored in tty.c. In the MPSAFE TTY code it is already stored
in tty_info.c. To make integration of the MPSAFE TTY code a little
easier, take the same approach.
This makes the TTY code a little bit more readable, because having the
proc_*/thread_* routines in tty.c is very distractful.
Approved by: philip (mentor)
ttyfree(), freeing the tty. Since destroy_dev() may call d_purge()
cdevsw method, that is the ttypurge() for the tty, the code ends up
accessing freed tty structure.
Put the ttyrel() after destroy_dev() in the ttyfree. To prevent the
panic the rev. 1.274 provided fix for, check the TS_GONE in sysctl
handler and refuse to provide information on such tty.
Reported, debugging help and tested by: pho
DIscussed with and reviewed by: jhb
MFC after: 1 week
in the giant_trick routines after the dev_refthread increments the
si_threadcount. Remove assert, do not perform dev_relthread() for failed
dev_refthread(), and handle failure in the tty_gettp() callers (cdevsw
tty methods).
Before kern_conf.c 1.210 and 1.211, the kernel usually paniced in the
giant_trick routines dereferencing NULL cdevsw, not taking this fault.
Reported by: Vince Hoffman <jhary unsane co uk>
Debugging help and tested by: pho
Reviewed by: jhb
MFC after: 1 week
For some reason, the <sys/tty.h> header file also contains routines of the
clists and console that are used inside the TTY layer. Because the clists
are not only used by the TTY layer (example: various input drivers), we'd
better move the entire clist programming interface into <sys/clist.h>. Also
remove a declaration of nonexistent variable.
The <sys/tty.h> header also contains various definitions for the console
code (tty_cons.c). Also move these to <sys/cons.h>, because they are
not implemented inside the TTY layer.
While there, create separate malloc pools for the clist and console code.
Approved by: philip (mentor)
requiring the per-process spinlock to only requiring the process lock.
- Reflect these changes in the proc.h documentation and consumers throughout
the kernel. This is a substantial reduction in locking cost for these
fields and was made possible by recent changes to threading support.
dev2udev() when a tty was being detached concurrently with the sysctl
handler:
- Hold the 'tty_list_mutex' lock while we read all the fields out of the
struct tty for copying out later. Previously the pty(4) and pts(4)
destroy routines could set t_dev to NULL, drop their reference on the
tty and destroy the cdev while the sysctl handler was attempting to
invoke dev2udev() on the cdev being destroyed. This happened when the
sysctl handler read the value of t_dev prior to it being set to NULL
either due to it being stale or due to timing races. By holding the
list lock we guarantee that the destroy routines will block in ttyrel()
in that case and not destroy the cdev until after we've copied all of our
data. We may see a NULL cdev pointer or we may see the previous value,
but the previous value will no longer point to a destroyed cdev if we
see it.
- Fix the ttyfree() routine used by tty device drivers in their detach
methods to use ttyrel() on the tty so we don't leak them. Also, fix it
to use the same order of operations as pty/pts destruction (set t_dev
NULL, ttyrel(), destroy_dev()) so it cooperates with the sysctl handler.
MFC after: 3 days
Tested by: avatar
filt_ttyrdetach() etc would later attempt to dereference cdev->si_tty,
causing a 0xdeadc0de dereference. Change kn_hook value from cdev to
struct tty to avoid dereferencing freed cdev.
In ttygone(), wake up select(), sigio and kevent() users in addition
to the queue sleepers.
Return EV_EOF from kevent filters if TS_GONE is set.
Submitted by: peter
Tested by: Peter Holm
Approved by: re (kensmith)
MFC after: 2 weeks
- Attempt to return the ttyinfo() selection algorithm to something sane
as it has been broken and disabled for some time. Adapt this algorithm
in such a way that it does not conflict with per-cpu scheduler locking.
Tested by: kris, current@
Tested on: i386, amd64, ULE, 4BSD, libthr, libkse, PREEMPTION, etc.
Discussed with: kris, attilio, kmacy, jhb, julian, bde (small parts each)
Make part of John Birrell's KSE patch permanent..
Specifically, remove:
Any reference of the ksegrp structure. This feature was
never fully utilised and made things overly complicated.
All code in the scheduler that tried to make threaded programs
fair to unthreaded programs. Libpthread processes will already
do this to some extent and libthr processes already disable it.
Also:
Since this makes such a big change to the scheduler(s), take the opportunity
to rename some structures and elements that had to be moved anyhow.
This makes the code a lot more readable.
The ULE scheduler compiles again but I have no idea if it works.
The 4bsd scheduler still reqires a little cleaning and some functions that now do
ALMOST nothing will go away, but I thought I'd do that as a separate commit.
Tested by David Xu, and Dan Eischen using libthr and libpthread.
specific privilege names to a broad range of privileges. These may
require some future tweaking.
Sponsored by: nCircle Network Security, Inc.
Obtained from: TrustedBSD Project
Discussed on: arch@
Reviewed (at least in part) by: mlaier, jmg, pjd, bde, ceri,
Alex Lyashkov <umka at sevcity dot net>,
Skip Ford <skip dot ford at verizon dot net>,
Antoine Brodin <antoine dot brodin at laposte dot net>
and opens a small race window before tp->t_session->s_leader is accessed. In case
tp->t_session has just been set to NULL elsewhere, we get a panic().
This fix is a bandaid until someone else fixes the whole locking in the tty subsystem.
Definitly more work needs to be done.
MFC after: 1 week
Reviewed by: mlaier
PR: kern/103101
to COMPAT_43TTY.
Add COMPAT_43TTY to NOTES and */conf/GENERIC
Compile tty_compat.c only under the new option.
Spit out
#warning "Old BSD tty API used, please upgrade."
if ioctl_compat.h gets #included from userland.
can't acquire an sx lock in ttyinfo() because ttyinfo() can be called
from interrupt handlers (such as atkbd_intr()). Instead, go back to
locking the process group while we pick a thread to display information for
and hold that lock until after we drop sched_lock to make sure the
process doesn't exit out from under us. sched_lock ensures that the
specific thread from that process doesn't go away. To protect against
the process exiting after we drop the proc lock but before we dereference
it to lookup the pid and p_comm in the call to ttyprintf(), we now copy
the pid and p_comm to local variables while holding the proc lock.
This problem was found by the recently added TD_NO_SLEEPING assertions for
interrupt handlers.
Tested by: emaste
MFC after: 1 week
- Introducing the possibility of using locks different than mutexes
for the knlist locking. In order to do this, we add three arguments to
knlist_init() to specify the functions to use to lock, unlock and
check if the lock is owned. If these arguments are NULL, we assume
mtx_lock, mtx_unlock and mtx_owned, respectively.
- Using the vnode lock for the knlist locking, when doing kqueue operations
on a vnode. This way, we don't have to lock the vnode while holding a
mutex, in filt_vfsread.
Reviewed by: jmg
Approved by: re (scottl), scottl (mentor override)
Pointyhat to: ssouhlal
Will be happy: everyone
guard against NULL t_modem entry. Otherwise, driver doesn't have t_modem
callback implemented(such like sys/dev/usb/ucycom.c) would panic when
someone opens the driver's associated tty device.
Reviewed by: phk, sam (mentor)
- Have TS_ZOMBIE ttys return POLLHUP instead of POLLERR
- Remove unneeded POLLWRNORM (old bug)
- TS_ZOMBIE ttys will set POLLIN and POLLRDNORM
- Do not call selrecord in TS_ZOMBIE ttys
PR: kern/73821
Reviewed by: bde
MFC after: 4 weeks
modes on a tty structure.
Both the ".init" and the current settings are initialized allowing
the function to be used both at attach and open time.
The function takes an argument to decide if echoing should be enabled.
Echoing should not be enabled for regular physical serial ports
unless they are consoles, in which case they should be configured
by ttyconsolemode() instead.
Use the new function throughout.
the raw values including for child process statistics and only compute the
system and user timevals on demand.
- Fix the various kern_wait() syscall wrappers to only pass in a rusage
pointer if they are going to use the result.
- Add a kern_getrusage() function for the ABI syscalls to use so that they
don't have to play stackgap games to call getrusage().
- Fix the svr4_sys_times() syscall to just call calcru() to calculate the
times it needs rather than calling getrusage() twice with associated
stackgap, etc.
- Add a new rusage_ext structure to store raw time stats such as tick counts
for user, system, and interrupt time as well as a bintime of the total
runtime. A new p_rux field in struct proc replaces the same inline fields
from struct proc (i.e. p_[isu]ticks, p_[isu]u, and p_runtime). A new p_crux
field in struct proc contains the "raw" child time usage statistics.
ruadd() has been changed to handle adding the associated rusage_ext
structures as well as the values in rusage. Effectively, the values in
rusage_ext replace the ru_utime and ru_stime values in struct rusage. These
two fields in struct rusage are no longer used in the kernel.
- calcru() has been split into a static worker function calcru1() that
calculates appropriate timevals for user and system time as well as updating
the rux_[isu]u fields of a passed in rusage_ext structure. calcru() uses a
copy of the process' p_rux structure to compute the timevals after updating
the runtime appropriately if any of the threads in that process are
currently executing. It also now only locks sched_lock internally while
doing the rux_runtime fixup. calcru() now only requires the caller to
hold the proc lock and calcru1() only requires the proc lock internally.
calcru() also no longer allows callers to ask for an interrupt timeval
since none of them actually did.
- calcru() now correctly handles threads executing on other CPUs.
- A new calccru() function computes the child system and user timevals by
calling calcru1() on p_crux. Note that this means that any code that wants
child times must now call this function rather than reading from p_cru
directly. This function also requires the proc lock.
- This finishes the locking for rusage and friends so some of the Giant locks
in exit1() and kern_wait() are now gone.
- The locking in ttyinfo() has been tweaked so that a shared lock of the
proctree lock is used to protect the process group rather than the process
group lock. By holding this lock until the end of the function we now
ensure that the process/thread that we pick to dump info about will no
longer vanish while we are trying to output its info to the console.
Submitted by: bde (mostly)
MFC after: 1 month
the new subr_unit.c code.
For now assert Giant in ttycreate() and ttyfree(). It is not obvious that
it will ever pay off to lock these with anything else.
generic way. This code will allow a similar amount of code to be
removed from most if not all serial port drivers.
Add generic cdevsw for tty devices.
Add generic slave cdevsw for init/lock devices.
Add ttypurge function which wakes up all know generic sleep
points in the tty code, and calls into the hw-driver if it
provides a method.
Add ttycreate function which creates tty device and optionally
cua device. In both cases .init/.lock devices are created
as well.
Change ttygone() slightly to also call the hw driver provided
purge routine.
Add ttyfree() which will purge and destroy the cdevs.
Add ttyconsole mode for setting console friendly termios
on a port.
most if not all of our tty drivers in the future.
Centralizing this stuff enables us to remove about 100 lines of
almost but not quite perfectly copy&paste code from each tty driver.
a more complete subsystem, and removes the knowlege of how things are
implemented from the drivers. Include locking around filter ops, so a
module like aio will know when not to be unloaded if there are outstanding
knotes using it's filter ops.
Currently, it uses the MTX_DUPOK even though it is not always safe to
aquire duplicate locks. Witness currently doesn't support the ability
to discover if a dup lock is ok (in some cases).
Reviewed by: green, rwatson (both earlier versions)
future:
rename ttyopen() -> tty_open() and ttyclose() -> tty_close().
We need the ttyopen() and ttyclose() for the new generic cdevsw
functions for tty devices in order to have consistent naming.
copies.
No current line disciplines have a dynamically changing hotchar, and
expecting to receive anything sensible during a change in ldisc is
insane so no locking of the hotchar field is necessary.
we have to revert to TTYDISC which we know will successfully open
rather than try the previous ldisc which might also fail to open.
Do not let ldisc implementations muck about with ->t_line, and remove
code which checks for reopens, it should never happen.
Move ldisc->l_hotchar to tty->t_hotchar and have ldisc implementation
initialize it in their open routines. Reset to zero when we enter
TTYDISC. ("no" should really be -1 since zero could be a valid
hotchar for certain old european mainframe protocols.)
Add two new functions: ttyref() and ttyrel(). ttymalloc() creates a struct
tty with a reference count of one. when ttyrel sees the count go to zero,
struct tty is freed.
Hold references for open ttys and for ttys which are controlling terminal
for sessions.
Until drivers start using ttyrel(), this commit will make no difference.
