[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 (file)
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -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;
 

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 9d65712..6a56ae4 100644 (file)
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -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 @@ restart:
        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 @@ restart:
         */
         
        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;
 }