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

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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@ab.jp.nec.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
This commit is contained in:
Kosuke Tatsukawa 2015-10-08 21:35:40 -03:00 committed by Mauro Carvalho Chehab
parent dcc7fdbec5
commit 5063452ecb
1 changed files with 1 additions and 2 deletions
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1
drivers/media/usb/cpia2/cpia2_usb.c
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@ -890,7 +890,6 @@ 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);
}