From c8e33141911bf8fe87dc6c92793b9a59b2be0130 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 14 Oct 2009 08:59:49 -0700
Subject: [PATCH] tty: Make flush_to_ldisc() locking more robust
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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>
---
drivers/char/tty_buffer.c | 29 +++++++++++++----------------
1 file 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);
--
2.34.1