brcmfmac: fix waitqueue_active without memory barrier in brcmfmac driver

brcmf_msgbuf_ioctl_resp_wake() 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.

  brcmf_msgbuf_ioctl_resp_wake		    brcmf_msgbuf_ioctl_resp_wait
------------------------------------------------------------------------
if (waitqueue_active(&msgbuf->ioctl_resp_wait))
/* The CPU might reorder the test for
   the waitqueue up here, before
   prior writes complete */
   	   	     	 	       /* wait_event_timeout */
				        /* __wait_event_timeout */
					 /* ___wait_event */
					 prepare_to_wait_event(&wq, &__wait,
					   state);
					 if (msgbuf->ctl_completed)
					 ...
msgbuf->ctl_completed = true;
					 schedule_timeout(__ret))
------------------------------------------------------------------------

There are three other place in drivers/net/wireless/brcm80211/brcmfmac/
which have similar code.  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).

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c
index ccde5599c401..23eaf0f71c43 100644
--- a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c
+++ b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c
@@ -196,8 +196,7 @@ static int brcmf_usb_ioctl_resp_wait(struct brcmf_usbdev_info *devinfo)
 
 static void brcmf_usb_ioctl_resp_wake(struct brcmf_usbdev_info *devinfo)
 {
-	if (waitqueue_active(&devinfo->ioctl_resp_wait))
-		wake_up(&devinfo->ioctl_resp_wait);
+	wake_up(&devinfo->ioctl_resp_wait);
 }
 
 static void