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Subject[PROBLEM]: race condition in tty subsystem
From
Date
This should be directed to the maintainer of the tty (serial)
subsystem, but I couldn't find who this is.


I'm seeing a race condition in the tty subsystem, canonical mode
processing, on multiprocessors.
Serial ports will get into a state, where a read on the port will
never block even though O_NONBLOCK is
not set. If there is no data for the port, (i.e. the majority of the
time), a read will immediately return
with 0. I think this is incorrect.

The version of the kernel that I'm running is 2.4.0-test11. I've also
seen it in 2.2 kernels. The module in
question, drivers/char/n_tty.c, is unchanged in the final version of
2.4.0.

This problem occurs fairly frequently on a dual-processor Itanium
(Lion B2). I've seen it also occur
on dual-processor x86 systems, but much less frequently (don't know
why).

To get into a bit more detail ....

When the serial port is in this state, the tty->canon_data field is
always non-zero. This indicates that
there is a canonical "chunk". However, there is no data to process.
This is what causes the read_chan
routine in n_tty.c to immediately return with 0 data. The port stays
this way until closed.

The problem, to me, is that the code that puts data into a tty's queue
and increments tty->canon_data
(n_tty_receive_char in n_tty.c) does not do this atomically. The code
that pulls data off the tty's queue and
decrements tty->canon_data (read_chan in n_tty.c) can occur at the
same time on a multiprocessor and
possibly cause canon_data to be inconsistent.

I have a test program that demonstrates this if the maintainer is
interested.

Here is the patch that I used to fix it ----

--- linux/drivers/char/n_tty.c Mon Jul 31 13:14:07 2000
+++ linux/drivers/char/n_tty.c-fix Wed Jan 10 07:46:43 2001
@@ -102,6 +102,15 @@
spin_unlock_irqrestore(&tty->read_lock, flags);
}

+static inline void put_tty_queue_nolock(unsigned char c, struct
tty_struct *tty)
+{
+ if (tty->read_cnt < N_TTY_BUF_SIZE) {
+ tty->read_buf[tty->read_head] = c;
+ tty->read_head = (tty->read_head + 1) & (N_TTY_BUF_SIZE-1);
+ tty->read_cnt++;
+ }
+}
+
/*
* Check whether to call the driver.unthrottle function.
* We test the TTY_THROTTLED bit first so that it always
@@ -499,6 +508,8 @@

static inline void n_tty_receive_char(struct tty_struct *tty,
unsigned char c)
{
+ unsigned long flags;
+
if (tty->raw) {
put_tty_queue(c, tty);
return;
@@ -651,10 +662,12 @@
put_tty_queue(c, tty);

handle_newline:
+ spin_lock_irqsave(&tty->read_lock, flags);
set_bit(tty->read_head, &tty->read_flags);
- put_tty_queue(c, tty);
+ put_tty_queue_nolock(c, tty);
tty->canon_head = tty->read_head;
tty->canon_data++;
+ spin_unlock_irqrestore(&tty->read_lock, flags);
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
if (waitqueue_active(&tty->read_wait))
wake_up_interruptible(&tty->read_wait);
Thanks,
Mike Straub

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