The pty driver forces ldisc flow control on, regardless of available
receive buffer space, so the writer can be woken whenever unthrottle
is called. However, this 'forced throttle' has performance
consequences, as multiple atomic operations are necessary to
unthrottle and perform the write wakeup for every input line (in
canonical mode).
Instead, short-circuit the unthrottle if the tty is a pty and perform
the write wakeup directly.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Prepare to special case pty flow control; avoid forward declaration.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Prepare for special handling of pty throttle/unthrottle; factor
flow control into helper functions.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Prepare to factor throttle and unthrottle into helper functions;
relocate chars_in_buffer() to avoid forward declaration.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
No tty driver modifies termios during throttle() or unthrottle().
Therefore, only read safety is required.
However, tty_throttle_safe and tty_unthrottle_safe must still be
mutually exclusive; introduce throttle_mutex for that purpose.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
If the read buffer indices are in the same cache-line, cpus will
contended over the cache-line (so called 'false sharing').
Separate the producer-published fields from the consumer-published
fields; document the locks relevant to each field.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
User-space read() can run concurrently with receiving from device;
waiting for receive_buf() to complete is not required.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
lnext escapes the next input character as a literal, and must
be reset when canonical mode changes (to avoid misinterpreting
a special character as a literal if canonical mode is changed
back again).
lnext is specifically not reset on a buffer flush so as to avoid
misinterpreting the next input character as a special character.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
n_tty has a single-producer/single-consumer input model;
use lockless publish instead.
Use termios_rwsem to exclude both consumer and producer while
changing or resetting buffer indices, eg., when flushing. Also,
claim exclusive termios_rwsem to safely retrieve the buffer
indices from a thread other than consumer or producer
(eg., TIOCINQ ioctl).
Note the read_tail is published _after_ clearing the newline
indicator in read_flags to avoid racing the producer.
Drop read_lock spinlock.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
canon_data represented the # of lines which had been copied
to the receive buffer but not yet copied to the user buffer.
The value was tested to determine if input was available in
canonical mode (and also to force input overrun if the
receive buffer was full but a newline had not been received).
However, the actual count was irrelevent; only whether it was
non-zero (meaning 'is there any input to transfer?'). This
shared count is unnecessary and unsafe with a lockless algorithm.
The same check is made by comparing canon_head with read_tail instead.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Use termios_rwsem to guarantee safe access to the termios values.
This is particularly important for N_TTY as changing certain termios
settings alters the mode of operation.
termios_rwsem must be dropped across throttle/unthrottle since
those functions claim the termios_rwsem exclusively (to guarantee
safe access to the termios and for mutual exclusion).
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Storing the read_cnt creates an unnecessary shared variable
between the single-producer (n_tty_receive_buf()) and the
single-consumer (n_tty_read()).
Compute read_cnt from head & tail instead of storing.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Wrap read_buf indices (read_head, read_tail, canon_head) at
max representable value, instead of at the N_TTY_BUF_SIZE. This step
is necessary to allow lockless reads of these shared variables
(by updating the variables atomically).
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Prepare for replacing read_cnt field with computed value.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
N_TTY .chars_in_buffer() method requires serialized access if
the current thread is not the single-consumer, n_tty_read().
Separate the internal interface; prepare for lockless read-side.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Instead of pushing one char per loop, pre-compute the data length
to copy and copy all at once.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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()
method.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Although the driver-side input path must update the available
buffer space, it should not reschedule itself. If space is still
available and the flip buffers are not empty, flush_to_ldisc()
will loop again.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
receive_room is used to control the amount of data the flip
buffer work can push to the read buffer. This update is unsafe:
CPU 0 | CPU 1
|
| n_tty_read()
| n_tty_set_room()
| left = <calc of space>
n_tty_receive_buf() |
<push data to buffer> |
n_tty_set_room() |
left = <calc of space> |
tty->receive_room = left |
| tty->receive_room = left
receive_room is now updated with a stale calculation of the
available buffer space, and the subsequent work loop will likely
overwrite unread data in the input buffer.
Update receive_room atomically with the calculation of the
available buffer space.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
minimum_to_wake is unique to N_TTY processing, and belongs in
per-ldisc data.
Add the ldisc method, ldisc_ops::fasync(), to notify line disciplines
when signal-driven I/O is enabled or disabled. When enabled for N_TTY
(by fcntl(F_SETFL, O_ASYNC)), blocking reader/polls will be woken
for any readable input. When disabled, blocking reader/polls are not
woken until the read buffer is full.
Canonical mode (L_ICANON(tty), n_tty_data::icanon) is not affected by
the minimum_to_wake setting.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
I meet emacs hang in start if I do the operation below:
1: echo 3 > /proc/sys/vm/drop_caches
2: emacs BigFile
3: Press CTRL-S follow 2 immediately
Then emacs hang on, CTRL-Q can't resume, the terminal
hang on, you can do nothing with this terminal except
close it.
The reason is before emacs takeover control the tty,
we use CTRL-S to XOFF it. Then when emacs takeover the
control, it may don't use the flow-control, so emacs hang.
This patch fix it.
This patch will fix a kind of strange tty relation hang problem,
I believe I meet it with vim in ssh, and also see below bug report:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=465823
Signed-off-by: Wang YanQing <udknight@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The driver is no longer unthrottled on buffer reset, so remove
comments that claim it is.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Factor the packet mode status change from n_tty_flush_buffer
for use by follow-on patch.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Flip buffer work must not be scheduled by the line discipline
after the line discipline has been halted; issue warning.
Note: drivers can still schedule flip buffer work.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Concurrent access to tty->pgrp must be protected with tty->ctrl_lock.
Also, as noted in the comments, reading current->signal->tty is
safe because either,
1) current->signal->tty is assigned by current, or
2) current->signal->tty is set to NULL.
NB: for reference, tty_check_change() implements a similar POSIX
check for the ioctls corresponding to tcflush(), tcdrain(),
tcsetattr(), tcsetpgrp(), tcflow() and tcsendbreak().
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
An ldisc reference is insufficient guarantee the foreground process
group is not in the process of being signalled from a hangup.
1) Reads of tty->pgrp must be locked with ctrl_lock
2) The group pid must be referenced for the duration of signalling.
Because the driver-side is not process-context, a pid reference
must be acquired.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
As noted in the following comment:
/* FIXME: there is a tiny race here if the receive room check runs
before the other work executes and empties the buffer (upping
the receiving room and unthrottling. We then throttle and get
stuck. This has been observed and traced down by Vincent Pillet/
We need to address this when we sort out out the rx path locking */
Use new safe throttle/unthrottle functions to re-evaluate conditions
if interrupted by the complement flow control function.
Reported-by: Vincent Pillet <vincentx.pillet@intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2-line function check_unthrottle() is now only called from
n_tty_read(); merge into caller.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The PPS (Pulse-Per-Second) line discipline has developed a number of
unhealthy attachments to core tty data and functions, ultimately leading
to its breakage.
The previous patches fixed the crashing. This one reduces coupling further
by eliminating the timestamp parameter from the dcd_change ldisc method.
This reduces header file linkage and makes the extension more generic,
and the timestamp read is delayed only slightly, from just before the
ldisc->ops->dcd_change method call to just after.
Fix attendant build breakage in
drivers/tty/n_tty.c
drivers/tty/tty_buffer.c
drivers/staging/speakup/selection.c
drivers/staging/dgrp/dgrp_*.c
Cc: William Hubbs <w.d.hubbs@gmail.com>
Cc: Chris Brannon <chris@the-brannons.com>
Cc: Kirk Reiser <kirk@braille.uwo.ca>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: George Spelvin <linux@horizon.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
The "normal" spin lock that guards the N_TTY line discipline read section
is replaced by a raw spin lock.
On a PREEMP_RT system this prevents unwanted scheduling overhead when data is
read at the same time as data is being received: while RX IRQ threaded handling
is busy a TTY read call is performed from a RT priority > threaded IRQ priority.
The read call tries to take the read section spin lock (held by the threaded
IRQ) which blocks and causes a context switch to/from the threaded IRQ handler
until the spin lock is unlocked.
Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
When the tty input buffer is full and thereby throttled,
flushing/resetting the read buffer should unthrottle to allow more
data to be received.
Signed-off-by: Karthik Manamcheri <Karthik.Manamcheri@ni.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
WARN_RATELIMIT() expects the warning to end with a newline if one
is needed.
Not doing so results in odd looking warnings such as:
[ 1339.454272] tty is NULLPid: 7147, comm: kworker/4:0 Tainted: G W 3.7.0-rc2-next-20121025-sasha-00001-g673f98e-dirty #75
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
So this is it. The big step why we did all the work over the past
kernel releases. Now everything is prepared, so nothing protects us
from doing that big step.
| | \ \ nnnn/^l | |
| | \ / / | |
| '-,.__ => \/ ,-` => | '-,.__
| O __.´´) ( .` | O __.´´)
~~~ ~~ `` ~~~ ~~
The buffers are now in the tty_port structure and we can start
teaching the buffer helpers (insert char/string, flip etc.) to use
tty_port instead of tty_struct all around.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
In some funtions we need only n_tty_data, so pass it down directly in
case tty is not needed there.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
atomic_write_lock is not n_tty specific, so move it up in the
tty_struct.
And since these are the last ones to move, remove also the comment
saying there are some ldisc' members. There are none now.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
All the ring-buffers...
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Here we move bitmaps and use DECLARE_BITMAP to declare them in the new
structure. And instead of memset, we use bitmap_zero as it is more
appropriate.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Here we start moving all the n_tty related bits from tty_struct to
the newly defined n_tty_data struct in n_tty proper.
In this patch primitive members and bits are moved. The rest will be
done per-partes in the next patches.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
All n_tty related members from tty_struct will be moved here.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This is a private member of n_tty. Stop accessing it. Instead, take is
as an argument.
This is needed to allow clean switch of the private members to a
separate private structure of n_tty.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Peter Hurley