Commit ff55fe20 authored by Jason Baron's avatar Jason Baron Committed by Linus Torvalds

[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: default avatarJason Baron <jbaron@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 69ac5964
......@@ -149,15 +149,14 @@ static int pty_write_room(struct tty_struct *tty)
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;
......
......@@ -469,21 +469,19 @@ static void tty_ldisc_enable(struct tty_struct *tty)
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 @@ static int tty_set_ldisc(struct tty_struct *tty, int ldisc)
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 @@ static int tty_set_ldisc(struct tty_struct *tty, int ldisc)
*/
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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