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, 18 insertions, 5 deletions

diff --git a/drivers/infiniband/hw/mthca/mthca_srq.c b/drivers/infiniband/hw/mthca/mthca_srq.c
index adcaf85355ae..1ea433291fa7 100644
--- a/drivers/infiniband/hw/mthca/mthca_srq.c
+++ b/drivers/infiniband/hw/mthca/mthca_srq.c
@@ -241,7 +241,7 @@ int mthca_alloc_srq(struct mthca_dev *dev, struct mthca_pd *pd,
 		goto err_out_mailbox;
 
 	spin_lock_init(&srq->lock);
-	atomic_set(&srq->refcount, 1);
+	srq->refcount = 1;
 	init_waitqueue_head(&srq->wait);
 
 	if (mthca_is_memfree(dev))
@@ -308,6 +308,17 @@ err_out:
 	return err;
 }
 
+static inline int get_srq_refcount(struct mthca_dev *dev, struct mthca_srq *srq)
+{
+	int c;
+
+	spin_lock_irq(&dev->srq_table.lock);
+	c = srq->refcount;
+	spin_unlock_irq(&dev->srq_table.lock);
+
+	return c;
+}
+
 void mthca_free_srq(struct mthca_dev *dev, struct mthca_srq *srq)
 {
 	struct mthca_mailbox *mailbox;
@@ -329,10 +340,10 @@ void mthca_free_srq(struct mthca_dev *dev, struct mthca_srq *srq)
 	spin_lock_irq(&dev->srq_table.lock);
 	mthca_array_clear(&dev->srq_table.srq,
 			  srq->srqn & (dev->limits.num_srqs - 1));
+	--srq->refcount;
 	spin_unlock_irq(&dev->srq_table.lock);
 
-	atomic_dec(&srq->refcount);
-	wait_event(srq->wait, !atomic_read(&srq->refcount));
+	wait_event(srq->wait, !get_srq_refcount(dev, srq));
 
 	if (!srq->ibsrq.uobject) {
 		mthca_free_srq_buf(dev, srq);
@@ -414,7 +425,7 @@ void mthca_srq_event(struct mthca_dev *dev, u32 srqn,
 	spin_lock(&dev->srq_table.lock);
 	srq = mthca_array_get(&dev->srq_table.srq, srqn & (dev->limits.num_srqs - 1));
 	if (srq)
-		atomic_inc(&srq->refcount);
+		++srq->refcount;
 	spin_unlock(&dev->srq_table.lock);
 
 	if (!srq) {
@@ -431,8 +442,10 @@ void mthca_srq_event(struct mthca_dev *dev, u32 srqn,
 	srq->ibsrq.event_handler(&event, srq->ibsrq.srq_context);
 
 out:
-	if (atomic_dec_and_test(&srq->refcount))
+	spin_lock(&dev->srq_table.lock);
+	if (!--srq->refcount)
 		wake_up(&srq->wait);
+	spin_unlock(&dev->srq_table.lock);
 }
 
 /*