hi,
This is a re-posting of a fix for a race condition between changing the
line discipline on a pty and and a poll on the 'other' side. The reference
is: http://marc.theaimsgroup.com/?l=linux-kernel&m=111342171410005&w=2.
Below is a diff against the current tree.
thanks,
-Jason
--- linux-2.6.12/drivers/char/pty.c 2005-06-17 15:48:29.000000000 -0400
+++ linux-2.6.12.working/drivers/char/pty.c 2005-07-11 12:21:03.000000000 -0400
@@ -149,15 +149,14 @@
static int pty_chars_in_buffer(struct tty_struct *tty)
{
struct tty_struct *to = tty->link;
- ssize_t (*chars_in_buffer)(struct tty_struct *);
int count;
/* We should get the line discipline lock for "tty->link" */
- if (!to || !(chars_in_buffer = to->ldisc.chars_in_buffer))
+ if (!to || !to->ldisc.chars_in_buffer)
return 0;
/* The ldisc must report 0 if no characters available to be read */
- count = chars_in_buffer(to);
+ count = to->ldisc.chars_in_buffer(to);
if (tty->driver->subtype == PTY_TYPE_SLAVE) return count;
--- linux-2.6.12/drivers/char/tty_io.c 2005-07-11 12:13:48.000000000 -0400
+++ linux-2.6.12.working/drivers/char/tty_io.c 2005-07-11 12:19:21.000000000 -0400
@@ -470,21 +470,19 @@
static int tty_set_ldisc(struct tty_struct *tty, int ldisc)
{
- int retval = 0;
- struct tty_ldisc o_ldisc;
+ int retval = 0;
+ struct tty_ldisc o_ldisc;
char buf[64];
int work;
unsigned long flags;
struct tty_ldisc *ld;
+ struct tty_struct *o_tty;
if ((ldisc < N_TTY) || (ldisc >= NR_LDISCS))
return -EINVAL;
restart:
- if (tty->ldisc.num == ldisc)
- return 0; /* We are already in the desired discipline */
-
ld = tty_ldisc_get(ldisc);
/* Eduardo Blanco <[email protected]> */
/* Cyrus Durgin <[email protected]> */
@@ -495,45 +493,74 @@
if (ld == NULL)
return -EINVAL;
- o_ldisc = tty->ldisc;
-
tty_wait_until_sent(tty, 0);
+ if (tty->ldisc.num == ldisc) {
+ tty_ldisc_put(ldisc);
+ return 0;
+ }
+
+ o_ldisc = tty->ldisc;
+ o_tty = tty->link;
+
/*
* Make sure we don't change while someone holds a
* reference to the line discipline. The TTY_LDISC bit
* prevents anyone taking a reference once it is clear.
* We need the lock to avoid racing reference takers.
*/
-
+
spin_lock_irqsave(&tty_ldisc_lock, flags);
- if(tty->ldisc.refcount)
- {
- /* Free the new ldisc we grabbed. Must drop the lock
- first. */
+ if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) {
+ if(tty->ldisc.refcount) {
+ /* Free the new ldisc we grabbed. Must drop the lock
+ first. */
+ spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+ tty_ldisc_put(ldisc);
+ /*
+ * There are several reasons we may be busy, including
+ * random momentary I/O traffic. We must therefore
+ * retry. We could distinguish between blocking ops
+ * and retries if we made tty_ldisc_wait() smarter. That
+ * is up for discussion.
+ */
+ if (wait_event_interruptible(tty_ldisc_wait, tty->ldisc.refcount == 0) < 0)
+ return -ERESTARTSYS;
+ goto restart;
+ }
+ if(o_tty && o_tty->ldisc.refcount) {
+ spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+ tty_ldisc_put(ldisc);
+ if (wait_event_interruptible(tty_ldisc_wait, o_tty->ldisc.refcount == 0) < 0)
+ return -ERESTARTSYS;
+ goto restart;
+ }
+ }
+
+ /* if the TTY_LDISC bit is set, then we are racing against another ldisc change */
+
+ if (!test_bit(TTY_LDISC, &tty->flags)) {
spin_unlock_irqrestore(&tty_ldisc_lock, flags);
tty_ldisc_put(ldisc);
- /*
- * There are several reasons we may be busy, including
- * random momentary I/O traffic. We must therefore
- * retry. We could distinguish between blocking ops
- * and retries if we made tty_ldisc_wait() smarter. That
- * is up for discussion.
- */
- if(wait_event_interruptible(tty_ldisc_wait, tty->ldisc.refcount == 0) < 0)
- return -ERESTARTSYS;
+ ld = tty_ldisc_ref_wait(tty);
+ tty_ldisc_deref(ld);
goto restart;
}
- clear_bit(TTY_LDISC, &tty->flags);
+
+ clear_bit(TTY_LDISC, &tty->flags);
clear_bit(TTY_DONT_FLIP, &tty->flags);
- spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-
+ if (o_tty) {
+ clear_bit(TTY_LDISC, &o_tty->flags);
+ clear_bit(TTY_DONT_FLIP, &o_tty->flags);
+ }
+ spin_unlock_irqrestore(&tty_ldisc_lock, flags);
+
/*
* From this point on we know nobody has an ldisc
* usage reference, nor can they obtain one until
* we say so later on.
*/
-
+
work = cancel_delayed_work(&tty->flip.work);
/*
* Wait for ->hangup_work and ->flip.work handlers to terminate
@@ -584,10 +611,12 @@
*/
tty_ldisc_enable(tty);
+ if (o_tty)
+ tty_ldisc_enable(o_tty);
/* Restart it in case no characters kick it off. Safe if
already running */
- if(work)
+ if (work)
schedule_delayed_work(&tty->flip.work, 1);
return retval;
}
-
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