linux-toradex.git/drivers/tty/tty_buffer.c, branch v3.10.78

tty: Fix race condition if flushing tty flip buffers

2013-04-10T00:05:02+00:00

As Ilya Zykov identified in his patch 'PROBLEM: Race condition in
tty buffer's function flush_to_ldisc()', a race condition exists
which allows a parallel flush_to_ldisc() to flush and free the tty
flip buffers while those buffers are in-use. For example,

  CPU 0                         |  CPU 1                                  |  CPU 2
                                | flush_to_ldisc()                        |
                                |  grab spin lock                         |
tty_buffer_flush()              |                                         | flush_to_ldisc()
 wait for spin lock             |                                         |  wait for spin lock
                                |   if (!test_and_set_bit(TTYP_FLUSHING)) |
                                |    while (next flip buffer)             |
                                |     ...                                 |
                                |     drop spin lock                      |
  grab spin lock                |                                         |
   if (test_bit(TTYP_FLUSHING)) |                                         |
    set_bit(TTYP_FLUSHPENDING)  |      receive_buf()                      |
    drop spin lock              |                                         |
                                |                                         |   grab spin lock
                                |                                         |    if (!test_and_set_bit(TTYP_FLUSHING))
                                |                                         |    if (test_bit(TTYP_FLUSHPENDING))
                                |                                         |    __tty_buffer_flush()

CPU 2 has just flushed and freed all tty flip buffers while CPU 1 is
transferring data from the head flip buffer.

The original patch was rejected under the assumption that parallel
flush_to_ldisc() was not possible. Because of necessary changes to
the workqueue api, work items can execute in parallel on SMP.

This patch differs slightly from the original patch by testing for
a pending flush _after_ each receive_buf(), since TTYP_FLUSHPENDING
can only be set while the lock is dropped around receive_buf().

Reported-by: Ilya Zykov 
Signed-off-by: Peter Hurley 
Acked-by: Ilya Zykov 
Cc: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

TTY: disable debugging warning

2013-03-04T08:52:09+00:00

We added a warning to flush_to_ldisc to report cases when it is called
with a NULL tty. It was for debugging purposes and it lead to a
patchset from Peter Hurley. The patchset however did not make it to
3.9, so disable the warning now to not disturb people.

We can re-add it when the series is in and we are hunting for another
bugs.

Reported-by: David Miller 
Cc: stable    # 3.8
Signed-off-by: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

pps: Move timestamp read into PPS code proper

2013-02-13T18:13:58+00:00

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 
Cc: Chris Brannon 
Cc: Kirk Reiser 
Cc: Samuel Thibault 
Signed-off-by: Peter Hurley 
Signed-off-by: George Spelvin 
Acked-by: Rodolfo Giometti 
Signed-off-by: Greg Kroah-Hartman

tty: Correct tty buffer flush.

2013-01-20T23:50:36+00:00

  The root of problem is carelessly zeroing pointer(in function __tty_buffer_flush()),
when another thread can use it. It can be cause of "NULL pointer dereference".
  Main idea of the patch, this is never free last (struct tty_buffer) in the active buffer.
Only flush the data for ldisc(buf->head->read = buf->head->commit).
At that moment driver can collect(write) data in buffer without conflict.
It is repeat behavior of flush_to_ldisc(), only without feeding data to ldisc.

Also revert:
  commit c56a00a165712fd73081f40044b1e64407bb1875
  tty: hold lock across tty buffer finding and buffer filling
In order to delete the unneeded locks any more.

Signed-off-by: Ilya Zykov 
CC: Alan Cox 
Signed-off-by: Greg Kroah-Hartman

TTY: switch tty_schedule_flip

2013-01-16T06:43:15+00:00

Now, we start converting tty buffer functions to actually use
tty_port. This will allow us to get rid of the need of tty in many
call sites. Only tty_port will needed and hence no more
tty_port_tty_get in those paths.

This is the last one: tty_schedule_flip

Signed-off-by: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

TTY: switch tty_flip_buffer_push

2013-01-16T06:30:15+00:00

Now, we start converting tty buffer functions to actually use
tty_port. This will allow us to get rid of the need of tty in many
call sites. Only tty_port will needed and hence no more
tty_port_tty_get in those paths.

Now, the one where most of tty_port_tty_get gets removed:
tty_flip_buffer_push.

IOW we also closed all the races in drivers not using tty_port_tty_get
at all yet.

Also we move tty_flip_buffer_push declaration from include/linux/tty.h
to include/linux/tty_flip.h to all others while we are changing it
anyway.

Signed-off-by: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

TTY: move low_latency to tty_port

2013-01-16T06:23:16+00:00

One point is to have less places where we actually need tty pointer.
The other is that low_latency is bound to buffer processing and
buffers are now in tty_port. So it makes sense to move low_latency to
tty_port too.

Signed-off-by: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

TTY: convert more flipping functions

2013-01-16T06:17:29+00:00

Now, we start converting tty buffer functions to actually use
tty_port. This will allow us to get rid of the need of tty pointer in
many call sites. Only tty_port will be needed and hence no more
tty_port_tty_get calls in those paths.

Now 4 string flipping ones are on turn:
* tty_insert_flip_string_flags
* tty_insert_flip_string_fixed_flag
* tty_prepare_flip_string
* tty_prepare_flip_string_flags

Signed-off-by: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

TTY: switch tty_buffer_request_room to tty_port

2013-01-16T06:17:28+00:00

Now, we start converting tty buffer functions to actually use
tty_port. This will allow us to get rid of the need of tty pointer in
many call sites. Only tty_port will be needed and hence no more
tty_port_tty_get calls in those paths.

Here we start with tty_buffer_request_room.

Signed-off-by: Jiri Slaby 
Signed-off-by: Greg Kroah-Hartman

tty: add missing newlines to WARN_RATELIMIT

2012-10-25T18:30:27+00:00

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 
Signed-off-by: Greg Kroah-Hartman