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pty: Rework the pty layer to use the normal buffering logic

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This fixes the ppp problems and various other issues with call locking
caused by one side of a pty called in one locking context trying to match
another with differing rules on the other side. We also get a big slack
space to work with that means we can bury the flow control deadlock case
for any conceivable real world situation.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Alan Cox 2009-07-07 16:39:41 +01:00 committed by Linus Torvalds
parent b4b21cac88
commit d945cb9cce
1 changed files with 59 additions and 95 deletions
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148
drivers/char/pty.c
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@ -75,114 +75,88 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
*/
static void pty_unthrottle(struct tty_struct *tty)
{
struct tty_struct *o_tty = tty->link;
if (!o_tty)
return;
tty_wakeup(o_tty);
tty_wakeup(tty->link);
set_bit(TTY_THROTTLED, &tty->flags);
}
/*
* WSH 05/24/97: modified to
* (1) use space in tty->flip instead of a shared temp buffer
* The flip buffers aren't being used for a pty, so there's lots
* of space available. The buffer is protected by a per-pty
* semaphore that should almost never come under contention.
* (2) avoid redundant copying for cases where count >> receive_room
* N.B. Calls from user space may now return an error code instead of
* a count.
/**
* pty_space - report space left for writing
* @to: tty we are writing into
*
* FIXME: Our pty_write method is called with our ldisc lock held but
* not our partners. We can't just wait on the other one blindly without
* risking deadlocks. At some point when everything has settled down we need
* to look into making pty_write at least able to sleep over an ldisc change.
*
* The return on no ldisc is a bit counter intuitive but the logic works
* like this. During an ldisc change the other end will flush its buffers. We
* thus return the full length which is identical to the case where we had
* proper locking and happened to queue the bytes just before the flush during
* the ldisc change.
* The tty buffers allow 64K but we sneak a peak and clip at 8K this
* allows a lot of overspill room for echo and other fun messes to
* be handled properly
*/
static int pty_space(struct tty_struct *to)
{
int n = 8192 - to->buf.memory_used;
if (n < 0)
return 0;
return n;
}
/**
* pty_write - write to a pty
* @tty: the tty we write from
* @buf: kernel buffer of data
* @count: bytes to write
*
* Our "hardware" write method. Data is coming from the ldisc which
* may be in a non sleeping state. We simply throw this at the other
* end of the link as if we were an IRQ handler receiving stuff for
* the other side of the pty/tty pair.
*/
static int pty_write(struct tty_struct *tty, const unsigned char *buf,
int count)
{
struct tty_struct *to = tty->link;
struct tty_ldisc *ld;
int c = count;
int c;
if (!to || tty->stopped)
if (tty->stopped)
return 0;
ld = tty_ldisc_ref(to);
if (ld) {
c = to->receive_room;
/* This isn't locked but our 8K is quite sloppy so no
big deal */
c = pty_space(to);
if (c > count)
c = count;
ld->ops->receive_buf(to, buf, NULL, c);
tty_ldisc_deref(ld);
if (c > 0) {
/* Stuff the data into the input queue of the other end */
c = tty_insert_flip_string(to, buf, c);
/* And shovel */
tty_flip_buffer_push(to);
tty_wakeup(tty);
}
return c;
}
/**
* pty_write_room - write space
* @tty: tty we are writing from
*
* Report how many bytes the ldisc can send into the queue for
* the other device.
*/
static int pty_write_room(struct tty_struct *tty)
{
struct tty_struct *to = tty->link;
if (!to || tty->stopped)
return 0;
return to->receive_room;
return pty_space(tty->link);
}
/*
* WSH 05/24/97: Modified for asymmetric MASTER/SLAVE behavior
* The chars_in_buffer() value is used by the ldisc select() function
* to hold off writing when chars_in_buffer > WAKEUP_CHARS (== 256).
* The pty driver chars_in_buffer() Master/Slave must behave differently:
/**
* pty_chars_in_buffer - characters currently in our tx queue
* @tty: our tty
*
* The Master side needs to allow typed-ahead commands to accumulate
* while being canonicalized, so we report "our buffer" as empty until
* some threshold is reached, and then report the count. (Any count >
* WAKEUP_CHARS is regarded by select() as "full".) To avoid deadlock
* the count returned must be 0 if no canonical data is available to be
* read. (The N_TTY ldisc.chars_in_buffer now knows this.)
*
* The Slave side passes all characters in raw mode to the Master side's
* buffer where they can be read immediately, so in this case we can
* return the true count in the buffer.
* Report how much we have in the transmit queue. As everything is
* instantly at the other end this is easy to implement.
*/
static int pty_chars_in_buffer(struct tty_struct *tty)
{
struct tty_struct *to = tty->link;
struct tty_ldisc *ld;
int count = 0;
/* We should get the line discipline lock for "tty->link" */
if (!to)
return 0;
/* We cannot take a sleeping reference here without deadlocking with
an ldisc change - but it doesn't really matter */
ld = tty_ldisc_ref(to);
if (ld == NULL)
return 0;
/* The ldisc must report 0 if no characters available to be read */
if (ld->ops->chars_in_buffer)
count = ld->ops->chars_in_buffer(to);
tty_ldisc_deref(ld);
if (tty->driver->subtype == PTY_TYPE_SLAVE)
return count;
/* Master side driver ... if the other side's read buffer is less than
* half full, return 0 to allow writers to proceed; otherwise return
* the count. This leaves a comfortable margin to avoid overflow,
* and still allows half a buffer's worth of typed-ahead commands.
*/
return (count < N_TTY_BUF_SIZE/2) ? 0 : count;
}
/* Set the lock flag on a pty */
@ -202,20 +176,10 @@ static void pty_flush_buffer(struct tty_struct *tty)
{
struct tty_struct *to = tty->link;
unsigned long flags;
struct tty_ldisc *ld;
if (!to)
return;
ld = tty_ldisc_ref(to);
/* The other end is changing discipline */
if (!ld)
return;
if (ld->ops->flush_buffer)
to->ldisc->ops->flush_buffer(to);
tty_ldisc_deref(ld);
/* tty_buffer_flush(to); FIXME */
if (to->packet) {
spin_lock_irqsave(&tty->ctrl_lock, flags);
tty->ctrl_status |= TIOCPKT_FLUSHWRITE;