linux-toradex.git/include/linux/tty_ldisc.h, branch v4.11-rc3

tty, n_tty: Remove fasync() ldisc notification

2016-01-28T19:58:02+00:00

Only the N_TTY line discipline implements the signal-driven i/o
notification enabled/disabled by fcntl(F_SETFL, O_ASYNC). The ldisc
fasync() notification is sent to the ldisc when the enable state has
changed (the tty core is notified via the fasync() VFS file operation).

The N_TTY line discipline used the enable state to change the wakeup
condition (minimum_to_wake = 1) for notifying the signal handler i/o is
available. However, just the presence of data is sufficient and necessary
to signal i/o is available, so changing minimum_to_wake is unnecessary
(and creates a race condition with read() and poll() which may be
concurrently updating minimum_to_wake).

Furthermore, since the kill_fasync() VFS helper performs no action if
the fasync list is empty, calling unconditionally is preferred; if
signal driven i/o just has been disabled, no signal will be sent by
kill_fasync() anyway so notification of the change via the ldisc
fasync() method is superfluous.

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman

tty: Remove chars_in_buffer() line discipline method

2016-01-27T23:01:44+00:00

The chars_in_buffer() line discipline method serves no functional
purpose, other than as a (dubious) debugging aid for mostly bit-rotting
drivers. Despite being documented as an optional method, every caller
is unconditionally executed (although conditionally compiled).
Furthermore, direct tty->ldisc access without an ldisc ref is unsafe.
Lastly, N_TTY's chars_in_buffer() has warned of removal since 3.12.

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman

tty_ldisc: add more limits to the @write_wakeup

2014-04-24T23:16:33+00:00

In the uart_handle_cts_change(), uart_write_wakeup() is called after
we call @uart_port->ops->start_tx().

The Documentation/serial/driver tells us:
-----------------------------------------------
  start_tx(port)
	Start transmitting characters.

	Locking: port->lock taken.
	Interrupts: locally disabled.
-----------------------------------------------

So when the uart_write_wakeup() is called, the port->lock is taken by
the upper. See the following callstack:

	|_ uart_write_wakeup
	   |_ tty_wakeup
	      |_ ld->ops->write_wakeup

With the port->lock held, we call the @write_wakeup. Some implemetation of
the @write_wakeup does not notice that the port->lock is held, and it still
tries to send data with uart_write() which will try to grab the prot->lock.
A dead lock occurs, see the following log caught in the Bluetooth by uart:

--------------------------------------------------------------------
BUG: spinlock lockup suspected on CPU#0, swapper/0/0
 lock: 0xdc3f4410, .magic: dead4ead, .owner: swapper/0/0, .owner_cpu: 0
CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W    3.10.17-16839-ge4a1bef #1320
[<80014cbc>] (unwind_backtrace+0x0/0x138) from [<8001251c>] (show_stack+0x10/0x14)
[<8001251c>] (show_stack+0x10/0x14) from [<802816ac>] (do_raw_spin_lock+0x108/0x184)
[<802816ac>] (do_raw_spin_lock+0x108/0x184) from [<806a22b0>] (_raw_spin_lock_irqsave+0x54/0x60)
[<806a22b0>] (_raw_spin_lock_irqsave+0x54/0x60) from [<802f5754>] (uart_write+0x38/0xe0)
[<802f5754>] (uart_write+0x38/0xe0) from [<80455270>] (hci_uart_tx_wakeup+0xa4/0x168)
[<80455270>] (hci_uart_tx_wakeup+0xa4/0x168) from [<802dab18>] (tty_wakeup+0x50/0x5c)
[<802dab18>] (tty_wakeup+0x50/0x5c) from [<802f81a4>] (imx_rtsint+0x50/0x80)
[<802f81a4>] (imx_rtsint+0x50/0x80) from [<802f88f4>] (imx_int+0x158/0x17c)
[<802f88f4>] (imx_int+0x158/0x17c) from [<8007abe0>] (handle_irq_event_percpu+0x50/0x194)
[<8007abe0>] (handle_irq_event_percpu+0x50/0x194) from [<8007ad60>] (handle_irq_event+0x3c/0x5c)
--------------------------------------------------------------------

This patch adds more limits to the @write_wakeup, the one who wants to
implemet the @write_wakeup should follow the limits which avoid the deadlock.

Signed-off-by: Huang Shijie 
Signed-off-by: Felipe Balbi 
Signed-off-by: Greg Kroah-Hartman

tty: tty_ldisc.h: Remove duplicate include

2014-02-18T20:50:26+00:00

linux/wait.h was included twice.

Signed-off-by: Sachin Kamat 
Signed-off-by: Greg Kroah-Hartman

tty: Always handle NULL flag ptr

2013-12-09T00:56:05+00:00

Most line disciplines already handle the undocumented NULL flag
ptr in their .receive_buf method; however, several don't.

Document the NULL flag ptr, and correct handling in the
N_MOUSE, N_GSM0710 and N_R394 line disciplines.

Signed-off-by: Peter Hurley 
Acked-by: Dmitry Torokhov 
Signed-off-by: Greg Kroah-Hartman

tty: Make ldisc input flow control concurrency-friendly

2013-07-23T23:42:59+00:00

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 
Signed-off-by: Greg Kroah-Hartman

tty: Replace ldisc locking with ldisc_sem

2013-07-23T23:38:34+00:00

Line discipline locking was performed with a combination of
a mutex, a status bit, a count, and a waitqueue -- basically,
a rw semaphore.

Replace the existing combination with an ld_semaphore.

Fixes:
 1) the 'reference acquire after ldisc locked' bug
 2) the over-complicated halt mechanism
 3) lock order wrt. tty_lock()
 4) dropping locks while changing ldisc
 5) previously unidentified deadlock while locking ldisc from
    both linked ttys concurrently
 6) previously unidentified recursive deadlocks

Adds much-needed lockdep diagnostics.

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman

n_tty: Encapsulate minimum_to_wake within N_TTY

2013-06-17T19:55:11+00:00

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 
Signed-off-by: Greg Kroah-Hartman

tty: Add timed, writer-prioritized rw semaphore

2013-05-20T19:30:32+00:00

The semantics of a rw semaphore are almost ideally suited
for tty line discipline lifetime management;  multiple active
threads obtain "references" (read locks) while performing i/o
to prevent the loss or change of the current line discipline
(write lock).

Unfortunately, the existing rw_semaphore is ill-suited in other
ways;
1) TIOCSETD ioctl (change line discipline) expects to return an
   error if the line discipline cannot be exclusively locked within
   5 secs. Lock wait timeouts are not supported by rwsem.
2) A tty hangup is expected to halt and scrap pending i/o, so
   exclusive locking must be prioritized.
   Writer priority is not supported by rwsem.

Add ld_semaphore which implements these requirements in a
semantically similar way to rw_semaphore.

Writer priority is handled by separate wait lists for readers and
writers. Pending write waits are priortized before existing read
waits and prevent further read locks.

Wait timeouts are trivially added, but obviously change the lock
semantics as lock attempts can fail (but only due to timeout).

This implementation incorporates the write-lock stealing work of
Michel Lespinasse .

Cc: Michel Lespinasse 
Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman

tty: Fix checkpatch errors in tty_ldisc.h

2013-03-18T23:14:55+00:00

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman