[PATCH] media: fix waitqueue_active without memory barrier in cpia2 driver

[Kosuke Tatsukawa]

cpia2_usb_disconnect() seems to be missing a memory barrier which might
cause the waker to not notice the waiter and miss sending a wake_up as
in the following figure.

	cpia2_usb_disconnect			sync
------------------------------------------------------------------------
					mutex_unlock(&cam->v4l2_lock);
if (waitqueue_active(&cam->wq_stream))
/* The CPU might reorder the test for
   the waitqueue up here, before
   prior writes complete */
					/* wait_event_interruptible */
					 /* __wait_event_interruptible */
					  /* ___wait_event */
					  long __int = prepare_to_wait_event(
					    &wq, &__wait, state);
					  if (!cam->streaming ||
					    frame->status == FRAME_READY)
cam->curbuff->status = FRAME_READY;
cam->curbuff->length = 0;
					  schedule()
------------------------------------------------------------------------

The attached patch removes the call to waitqueue_active() leaving just
wake_up() behind.  This fixes the problem because the call to
spin_lock_irqsave() in wake_up() will be an ACQUIRE operation.

I found this issue when I was looking through the linux source code
for places calling waitqueue_active() before wake_up*(), but without
preceding memory barriers, after sending a patch to fix a similar
issue in drivers/tty/n_tty.c  (Details about the original issue can be
found here: https://lkml.org/lkml/2015/9/28/849).

Signed-off-by: Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx>
---
 drivers/media/usb/cpia2/cpia2_usb.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/cpia2/cpia2_usb.c b/drivers/media/usb/cpia2/cpia2_usb.c
index 351a78a..c1aa1ab 100644
--- a/drivers/media/usb/cpia2/cpia2_usb.c
+++ b/drivers/media/usb/cpia2/cpia2_usb.c
@@ -890,8 +890,7 @@ static void cpia2_usb_disconnect(struct usb_interface *intf)
 		DBG("Wakeup waiting processes\n");
 		cam->curbuff->status = FRAME_READY;
 		cam->curbuff->length = 0;
-		if (waitqueue_active(&cam->wq_stream))
-			wake_up_interruptible(&cam->wq_stream);
+		wake_up_interruptible(&cam->wq_stream);
 	}
 
 	DBG("Releasing interface\n");
-- 
1.7.1
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