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authorPeter Hurley <peter@hurleysoftware.com>2013-03-20 13:20:43 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-04-09 17:05:02 -0700
commit39f610e40eecc39e4c34e047fd6904ca6b525520 (patch)
tree39af27120456a3a3b68b7068d65a71f5614db008 /drivers/tty
parent31bdfc649f4577c9b1120c67b7b1e85f7777aad0 (diff)
downloadop-kernel-dev-39f610e40eecc39e4c34e047fd6904ca6b525520.zip
op-kernel-dev-39f610e40eecc39e4c34e047fd6904ca6b525520.tar.gz
tty: Fix race condition if flushing tty flip buffers
As Ilya Zykov identified in his patch 'PROBLEM: Race condition in tty buffer's function flush_to_ldisc()', a race condition exists which allows a parallel flush_to_ldisc() to flush and free the tty flip buffers while those buffers are in-use. For example, CPU 0 | CPU 1 | CPU 2 | flush_to_ldisc() | | grab spin lock | tty_buffer_flush() | | flush_to_ldisc() wait for spin lock | | wait for spin lock | if (!test_and_set_bit(TTYP_FLUSHING)) | | while (next flip buffer) | | ... | | drop spin lock | grab spin lock | | if (test_bit(TTYP_FLUSHING)) | | set_bit(TTYP_FLUSHPENDING) | receive_buf() | drop spin lock | | | | grab spin lock | | if (!test_and_set_bit(TTYP_FLUSHING)) | | if (test_bit(TTYP_FLUSHPENDING)) | | __tty_buffer_flush() CPU 2 has just flushed and freed all tty flip buffers while CPU 1 is transferring data from the head flip buffer. The original patch was rejected under the assumption that parallel flush_to_ldisc() was not possible. Because of necessary changes to the workqueue api, work items can execute in parallel on SMP. This patch differs slightly from the original patch by testing for a pending flush _after_ each receive_buf(), since TTYP_FLUSHPENDING can only be set while the lock is dropped around receive_buf(). Reported-by: Ilya Zykov <linux@izyk.ru> Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Ilya Zykov <linux@izyk.ru> Cc: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/tty')
-rw-r--r--drivers/tty/tty_buffer.c22
1 files changed, 10 insertions, 12 deletions
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 578aa75..9121c1f 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -449,11 +449,6 @@ static void flush_to_ldisc(struct work_struct *work)
tty_buffer_free(port, head);
continue;
}
- /* Ldisc or user is trying to flush the buffers
- we are feeding to the ldisc, stop feeding the
- line discipline as we want to empty the queue */
- if (test_bit(TTYP_FLUSHPENDING, &port->iflags))
- break;
if (!tty->receive_room)
break;
if (count > tty->receive_room)
@@ -465,17 +460,20 @@ static void flush_to_ldisc(struct work_struct *work)
disc->ops->receive_buf(tty, char_buf,
flag_buf, count);
spin_lock_irqsave(&buf->lock, flags);
+ /* Ldisc or user is trying to flush the buffers.
+ 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(TTYP_FLUSHPENDING, &port->iflags)) {
+ __tty_buffer_flush(port);
+ clear_bit(TTYP_FLUSHPENDING, &port->iflags);
+ wake_up(&tty->read_wait);
+ break;
+ }
}
clear_bit(TTYP_FLUSHING, &port->iflags);
}
- /* 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(TTYP_FLUSHPENDING, &port->iflags)) {
- __tty_buffer_flush(port);
- clear_bit(TTYP_FLUSHPENDING, &port->iflags);
- wake_up(&tty->read_wait);
- }
spin_unlock_irqrestore(&buf->lock, flags);
tty_ldisc_deref(disc);
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