[PATCH] pty_chars_in_buffer oops fix
The idea of this patch is to lock both sides of a ptmx/pty pair during line
discipline changing. This is needed to ensure that say a poll on one side of
the pty doesn't occur while the line discipline is actively being changed.
This resulted in an oops reported on lkml, see:
http://marc.theaimsgroup.com/?l=linux-kernel&m=111342171410005&w=2
A 'hacky' approach was previously implmemented which served to eliminate the
poll vs. line discipline changing race. However, this patch takes a more
general approach to the issue. The patch only adds locking on a less often
used path, the line-discipline changing path, as opposed to locking the
ptmx/pty pair on read/write/poll paths.
The patch below, takes both ldisc locks in either order b/c the locks are both
taken under the same spinlock(). I thought about locking the ptmx/pty
separately, such as master always first but that introduces a 3 way deadlock.
For example, process 1 does a blocking read on the slave side. Then, process
2 does an ldisc change on the slave side, which acquires the master ldisc lock
but not the slave's. Finally, process 3 does a write which blocks on the
process 2's ldisc reference.
This patch does introduce some changes in semantics. For example, a line
discipline change on side 'a' of a ptmx/pty pair, will now wait for a
read/write to complete on the other side, or side 'b'. The current behavior
is to simply wait for any read/writes on only side 'a', not both sides 'a' and
'b'. I think this behavior makes sense, but I wanted to point it out.
I've tested the patch with a bunch of read/write/poll while changing the line
discipline out from underneath.
This patch obviates the need for the above "hide the problem" patch.
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index da32889..49f3997 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -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;
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 9d65712..6a56ae4 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -469,21 +469,19 @@
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 <ejbs@cs.cs.com.uy> */
/* Cyrus Durgin <cider@speakeasy.org> */
@@ -494,45 +492,74 @@
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);
+ 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
@@ -583,10 +610,12 @@
*/
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;
}
