tty: Make flush_to_ldisc() locking more robust

The locking logic in this function is extremely subtle, and it broke
when we started doing potentially concurrent 'flush_to_ldisc()' calls in
commit e043e42bdb66885b3ac10d27a01ccb9972e2b0a3 ("pty: avoid forcing
'low_latency' tty flag").

The code in flush_to_ldisc() used to set 'tty->buf.head' to NULL, with
the intention that this would then cause any other concurrent calls to
not do anything (locking note: we have to drop the buf.lock over the
call to ->receive_buf that can block, which is why we can have
concurrency here at all in the first place).

It also used to set the TTY_FLUSHING bit, which would then cause any
concurrent 'tty_buffer_flush()' to not free all the tty buffers and
clear 'tty->buf.tail'.  And with 'buf.head' being NULL, and 'buf.tail'
being non-NULL, new data would never touch 'buf.head'.

Does that sound a bit too subtle? It was.  If another concurrent call to
'flush_to_ldisc()' were to come in, the NULL buf.head would indeed cause
it to not process the buffer list, but it would still clear TTY_FLUSHING
afterwards, making the buffer protection against 'tty_buffer_flush()' no
longer work.

So this clears it all up.  We depend purely on TTY_FLUSHING for handling
re-entrancy, and stop playing games with the buffer list entirely.  In
fact, the buffer list handling is now robust enough that we could
probably stop doing the whole "protect against 'tty_buffer_flush()'"
thing entirely.

However, Alan also points out that we would probably be better off
simplifying the locking even further, and just take the tty ldisc_mutex
around all the buffer flushing calls.  That seems like a good idea, but
in the meantime this is a conceptually minimal fix (with the patch
itself being bigger than required just to clean the code up and make it
readable).

This fixes keyboard trouble under X:

	http://bugzilla.kernel.org/show_bug.cgi?id=14388

Reported-and-tested-by: Frédéric Meunier <fredlwm@gmail.com>
Reported-and-tested-by: Boyan <btanastasov@yahoo.co.uk>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 16 deletions

diff --git a/drivers/char/tty_buffer.c b/drivers/char/tty_buffer.c
index 3108991c5c8b..0296612cc7df 100644
--- a/drivers/char/tty_buffer.c
+++ b/drivers/char/tty_buffer.c
@@ -402,28 +402,26 @@ static void flush_to_ldisc(struct work_struct *work)
                 container_of(work, struct tty_struct, buf.work.work);
         unsigned long   flags;
         struct tty_ldisc *disc;
-        struct tty_buffer *tbuf, *head;
-        char *char_buf;
-        unsigned char *flag_buf;
 
         disc = tty_ldisc_ref(tty);
         if (disc == NULL)       /*  !TTY_LDISC */
                 return;
 
         spin_lock_irqsave(&tty->buf.lock, flags);
-        /* So we know a flush is running */
-        set_bit(TTY_FLUSHING, &tty->flags);
-        head = tty->buf.head;
-        if (head != NULL) {
-                tty->buf.head = NULL;
-                for (;;) {
-                        int count = head->commit - head->read;
+
+        if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) {
+                struct tty_buffer *head;
+                while ((head = tty->buf.head) != NULL) {
+                        int count;
+                        char *char_buf;
+                        unsigned char *flag_buf;
+
+                        count = head->commit - head->read;
                         if (!count) {
                                 if (head->next == NULL)
                                         break;
-                                tbuf = head;
-                                head = head->next;
-                                tty_buffer_free(tty, tbuf);
+                                tty->buf.head = head->next;
+                                tty_buffer_free(tty, head);
                                 continue;
                         }
                         /* Ldisc or user is trying to flush the buffers
@@ -445,9 +443,9 @@ static void flush_to_ldisc(struct work_struct *work)
                                                         flag_buf, count);
                         spin_lock_irqsave(&tty->buf.lock, flags);
                 }
-                /* Restore the queue head */
-                tty->buf.head = head;
+                clear_bit(TTY_FLUSHING, &tty->flags);
         }
+
         /* We may have a deferred request to flush the input buffer,
            if so pull the chain under the lock and empty the queue */
         if (test_bit(TTY_FLUSHPENDING, &tty->flags)) {
@@ -455,7 +453,6 @@ static void flush_to_ldisc(struct work_struct *work)
                 clear_bit(TTY_FLUSHPENDING, &tty->flags);
                 wake_up(&tty->read_wait);
         }
-        clear_bit(TTY_FLUSHING, &tty->flags);
         spin_unlock_irqrestore(&tty->buf.lock, flags);
 
         tty_ldisc_deref(disc);