IB/mthca: Fix race in reference counting

Fix races in in destroying various objects.  If a destroy routine
waits for an object to become free by doing

	wait_event(&obj->wait, !atomic_read(&obj->refcount));
	/* now clean up and destroy the object */

and another place drops a reference to the object by doing

	if (atomic_dec_and_test(&obj->refcount))
		wake_up(&obj->wait);

then this is susceptible to a race where the wait_event() and final
freeing of the object occur between the atomic_dec_and_test() and the
wake_up().  And this is a use-after-free, since wake_up() will be
called on part of the already-freed object.

Fix this in mthca by replacing the atomic_t refcounts with plain old
integers protected by a spinlock.  This makes it possible to do the
decrement of the reference count and the wake_up() so that it appears
as a single atomic operation to the code waiting on the wait queue.

While touching this code, also simplify mthca_cq_clean(): the CQ being
cleaned cannot go away, because it still has a QP attached to it.  So
there's no reason to be paranoid and look up the CQ by number; it's
perfectly safe to use the pointer that the callers already have.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

1 files changed, 21 insertions, 20 deletions

diff --git a/drivers/infiniband/hw/mthca/mthca_cq.c b/drivers/infiniband/hw/mthca/mthca_cq.c
index 312cf90731e..205854e9c66 100644
--- a/drivers/infiniband/hw/mthca/mthca_cq.c
+++ b/drivers/infiniband/hw/mthca/mthca_cq.c
@@ -238,9 +238,9 @@ void mthca_cq_event(struct mthca_dev *dev, u32 cqn,
 	spin_lock(&dev->cq_table.lock);
 
 	cq = mthca_array_get(&dev->cq_table.cq, cqn & (dev->limits.num_cqs - 1));
-
 	if (cq)
-		atomic_inc(&cq->refcount);
+		++cq->refcount;
+
 	spin_unlock(&dev->cq_table.lock);
 
 	if (!cq) {
@@ -254,8 +254,10 @@ void mthca_cq_event(struct mthca_dev *dev, u32 cqn,
 	if (cq->ibcq.event_handler)
 		cq->ibcq.event_handler(&event, cq->ibcq.cq_context);
 
-	if (atomic_dec_and_test(&cq->refcount))
+	spin_lock(&dev->cq_table.lock);
+	if (!--cq->refcount)
 		wake_up(&cq->wait);
+	spin_unlock(&dev->cq_table.lock);
 }
 
 static inline int is_recv_cqe(struct mthca_cqe *cqe)
@@ -267,23 +269,13 @@ static inline int is_recv_cqe(struct mthca_cqe *cqe)
 		return !(cqe->is_send & 0x80);
 }
 
-void mthca_cq_clean(struct mthca_dev *dev, u32 cqn, u32 qpn,
+void mthca_cq_clean(struct mthca_dev *dev, struct mthca_cq *cq, u32 qpn,
 		    struct mthca_srq *srq)
 {
-	struct mthca_cq *cq;
 	struct mthca_cqe *cqe;
 	u32 prod_index;
 	int nfreed = 0;
 
-	spin_lock_irq(&dev->cq_table.lock);
-	cq = mthca_array_get(&dev->cq_table.cq, cqn & (dev->limits.num_cqs - 1));
-	if (cq)
-		atomic_inc(&cq->refcount);
-	spin_unlock_irq(&dev->cq_table.lock);
-
-	if (!cq)
-		return;
-
 	spin_lock_irq(&cq->lock);
 
 	/*
@@ -301,7 +293,7 @@ void mthca_cq_clean(struct mthca_dev *dev, u32 cqn, u32 qpn,
 
 	if (0)
 		mthca_dbg(dev, "Cleaning QPN %06x from CQN %06x; ci %d, pi %d\n",
-			  qpn, cqn, cq->cons_index, prod_index);
+			  qpn, cq->cqn, cq->cons_index, prod_index);
 
 	/*
 	 * Now sweep backwards through the CQ, removing CQ entries
@@ -325,8 +317,6 @@ void mthca_cq_clean(struct mthca_dev *dev, u32 cqn, u32 qpn,
 	}
 
 	spin_unlock_irq(&cq->lock);
-	if (atomic_dec_and_test(&cq->refcount))
-		wake_up(&cq->wait);
 }
 
 void mthca_cq_resize_copy_cqes(struct mthca_cq *cq)
@@ -821,7 +811,7 @@ int mthca_init_cq(struct mthca_dev *dev, int nent,
 	}
 
 	spin_lock_init(&cq->lock);
-	atomic_set(&cq->refcount, 1);
+	cq->refcount = 1;
 	init_waitqueue_head(&cq->wait);
 
 	memset(cq_context, 0, sizeof *cq_context);
@@ -896,6 +886,17 @@ err_out:
 	return err;
 }
 
+static inline int get_cq_refcount(struct mthca_dev *dev, struct mthca_cq *cq)
+{
+	int c;
+
+	spin_lock_irq(&dev->cq_table.lock);
+	c = cq->refcount;
+	spin_unlock_irq(&dev->cq_table.lock);
+
+	return c;
+}
+
 void mthca_free_cq(struct mthca_dev *dev,
 		   struct mthca_cq *cq)
 {
@@ -929,6 +930,7 @@ void mthca_free_cq(struct mthca_dev *dev,
 	spin_lock_irq(&dev->cq_table.lock);
 	mthca_array_clear(&dev->cq_table.cq,
 			  cq->cqn & (dev->limits.num_cqs - 1));
+	--cq->refcount;
 	spin_unlock_irq(&dev->cq_table.lock);
 
 	if (dev->mthca_flags & MTHCA_FLAG_MSI_X)
@@ -936,8 +938,7 @@ void mthca_free_cq(struct mthca_dev *dev,
 	else
 		synchronize_irq(dev->pdev->irq);
 
-	atomic_dec(&cq->refcount);
-	wait_event(cq->wait, !atomic_read(&cq->refcount));
+	wait_event(cq->wait, !get_cq_refcount(dev, cq));
 
 	if (cq->is_kernel) {
 		mthca_free_cq_buf(dev, &cq->buf, cq->ibcq.cqe);