[PATCH] mei: fix waitqueue_active without memory barrier in mei drivers

[Kosuke Tatsukawa]

mei_cl_complete() 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.

	mei_cl_complete				mei_cl_write
------------------------------------------------------------------------
					mutex_unlock(&dev->device_lock);
...
if (waitqueue_active(&cl->tx_wait)) {
/* 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 (cl->writing_state == MEI_WRITE_COMPLETE)
cl->writing_state = MEI_WRITE_COMPLETE;
} else {
...
------------------------------------------------------------------------

There are three other place in drivers/misc/mei/ which have similar
code.  The attached patch either adds the missing memory barrier, or
removes the call to waitqueue_active() leaving just wake_up() behind.
The latter 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/misc/mei/client.c |   10 ++++++++++
 drivers/misc/mei/hw-me.c  |    3 +--
 drivers/misc/mei/hw-txe.c |    3 +--
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/mei/client.c b/drivers/misc/mei/client.c
index a6c87c7..546aa2b 100644
--- a/drivers/misc/mei/client.c
+++ b/drivers/misc/mei/client.c
@@ -1711,6 +1711,11 @@ void mei_cl_complete(struct mei_cl *cl, struct mei_cl_cb *cb)
 	case MEI_FOP_WRITE:
 		mei_io_cb_free(cb);
 		cl->writing_state = MEI_WRITE_COMPLETE;
+		/*
+		 * Memory barrier is required here to make sure change to
+		 * cl->writing_state is visible from other CPUs.
+		 */
+		smp_mb();
 		if (waitqueue_active(&cl->tx_wait)) {
 			wake_up_interruptible(&cl->tx_wait);
 		} else {
@@ -1721,6 +1726,11 @@ void mei_cl_complete(struct mei_cl *cl, struct mei_cl_cb *cb)
 
 	case MEI_FOP_READ:
 		list_add_tail(&cb->list, &cl->rd_completed);
+		/*
+		 * Memory barrier is required here to make sure change to
+		 * cl->rd_completed is visible from other CPUs.
+		 */
+		smp_mb();
 		if (waitqueue_active(&cl->rx_wait))
 			wake_up_interruptible_all(&cl->rx_wait);
 		else
diff --git a/drivers/misc/mei/hw-me.c b/drivers/misc/mei/hw-me.c
index 65511d3..0bc45ef 100644
--- a/drivers/misc/mei/hw-me.c
+++ b/drivers/misc/mei/hw-me.c
@@ -943,8 +943,7 @@ static void mei_me_pg_legacy_intr(struct mei_device *dev)
 
 	dev->pg_event = MEI_PG_EVENT_INTR_RECEIVED;
 	hw->pg_state = MEI_PG_OFF;
-	if (waitqueue_active(&dev->wait_pg))
-		wake_up(&dev->wait_pg);
+	wake_up(&dev->wait_pg);
 }
 
 /**
diff --git a/drivers/misc/mei/hw-txe.c b/drivers/misc/mei/hw-txe.c
index bae680c..b9ce17d 100644
--- a/drivers/misc/mei/hw-txe.c
+++ b/drivers/misc/mei/hw-txe.c
@@ -1091,8 +1091,7 @@ irqreturn_t mei_txe_irq_thread_handler(int irq, void *dev_id)
 		dev_dbg(dev->dev,
 			"Aliveness Interrupt: Status: %d\n", hw->aliveness);
 		dev->pg_event = MEI_PG_EVENT_RECEIVED;
-		if (waitqueue_active(&hw->wait_aliveness_resp))
-			wake_up(&hw->wait_aliveness_resp);
+		wake_up(&hw->wait_aliveness_resp);
 	}
 
 
-- 
1.7.1
--
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