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, 22 insertions, 9 deletions

diff --git a/drivers/infiniband/hw/mthca/mthca_qp.c b/drivers/infiniband/hw/mthca/mthca_qp.c
index f37b0e367323..19765f6f8d58 100644
--- a/drivers/infiniband/hw/mthca/mthca_qp.c
+++ b/drivers/infiniband/hw/mthca/mthca_qp.c
@@ -240,7 +240,7 @@ void mthca_qp_event(struct mthca_dev *dev, u32 qpn,
         spin_lock(&dev->qp_table.lock);
         qp = mthca_array_get(&dev->qp_table.qp, qpn & (dev->limits.num_qps - 1));
         if (qp)
-                atomic_inc(&qp->refcount);
+                ++qp->refcount;
         spin_unlock(&dev->qp_table.lock);
 
         if (!qp) {
@@ -257,8 +257,10 @@ void mthca_qp_event(struct mthca_dev *dev, u32 qpn,
         if (qp->ibqp.event_handler)
                 qp->ibqp.event_handler(&event, qp->ibqp.qp_context);
 
-        if (atomic_dec_and_test(&qp->refcount))
+        spin_lock(&dev->qp_table.lock);
+        if (!--qp->refcount)
                 wake_up(&qp->wait);
+        spin_unlock(&dev->qp_table.lock);
 }
 
 static int to_mthca_state(enum ib_qp_state ib_state)
@@ -833,10 +835,10 @@ int mthca_modify_qp(struct ib_qp *ibqp, struct ib_qp_attr *attr, int attr_mask)
          * entries and reinitialize the QP.
          */
         if (new_state == IB_QPS_RESET && !qp->ibqp.uobject) {
-                mthca_cq_clean(dev, to_mcq(qp->ibqp.send_cq)->cqn, qp->qpn,
+                mthca_cq_clean(dev, to_mcq(qp->ibqp.send_cq), qp->qpn,
                                qp->ibqp.srq ? to_msrq(qp->ibqp.srq) : NULL);
                 if (qp->ibqp.send_cq != qp->ibqp.recv_cq)
-                        mthca_cq_clean(dev, to_mcq(qp->ibqp.recv_cq)->cqn, qp->qpn,
+                        mthca_cq_clean(dev, to_mcq(qp->ibqp.recv_cq), qp->qpn,
                                        qp->ibqp.srq ? to_msrq(qp->ibqp.srq) : NULL);
 
                 mthca_wq_init(&qp->sq);
@@ -1096,7 +1098,7 @@ static int mthca_alloc_qp_common(struct mthca_dev *dev,
         int ret;
         int i;
 
-        atomic_set(&qp->refcount, 1);
+        qp->refcount = 1;
         init_waitqueue_head(&qp->wait);
         qp->state        = IB_QPS_RESET;
         qp->atomic_rd_en = 0;
@@ -1318,6 +1320,17 @@ int mthca_alloc_sqp(struct mthca_dev *dev,
         return err;
 }
 
+static inline int get_qp_refcount(struct mthca_dev *dev, struct mthca_qp *qp)
+{
+        int c;
+
+        spin_lock_irq(&dev->qp_table.lock);
+        c = qp->refcount;
+        spin_unlock_irq(&dev->qp_table.lock);
+
+        return c;
+}
+
 void mthca_free_qp(struct mthca_dev *dev,
                    struct mthca_qp *qp)
 {
@@ -1339,14 +1352,14 @@ void mthca_free_qp(struct mthca_dev *dev,
         spin_lock(&dev->qp_table.lock);
         mthca_array_clear(&dev->qp_table.qp,
                           qp->qpn & (dev->limits.num_qps - 1));
+        --qp->refcount;
         spin_unlock(&dev->qp_table.lock);
 
         if (send_cq != recv_cq)
                 spin_unlock(&recv_cq->lock);
         spin_unlock_irq(&send_cq->lock);
 
-        atomic_dec(&qp->refcount);
-        wait_event(qp->wait, !atomic_read(&qp->refcount));
+        wait_event(qp->wait, !get_qp_refcount(dev, qp));
 
         if (qp->state != IB_QPS_RESET)
                 mthca_MODIFY_QP(dev, qp->state, IB_QPS_RESET, qp->qpn, 0,
@@ -1358,10 +1371,10 @@ void mthca_free_qp(struct mthca_dev *dev,
          * unref the mem-free tables and free the QPN in our table.
          */
         if (!qp->ibqp.uobject) {
-                mthca_cq_clean(dev, to_mcq(qp->ibqp.send_cq)->cqn, qp->qpn,
+                mthca_cq_clean(dev, to_mcq(qp->ibqp.send_cq), qp->qpn,
                                qp->ibqp.srq ? to_msrq(qp->ibqp.srq) : NULL);
                 if (qp->ibqp.send_cq != qp->ibqp.recv_cq)
-                        mthca_cq_clean(dev, to_mcq(qp->ibqp.recv_cq)->cqn, qp->qpn,
+                        mthca_cq_clean(dev, to_mcq(qp->ibqp.recv_cq), qp->qpn,
                                        qp->ibqp.srq ? to_msrq(qp->ibqp.srq) : NULL);
 
                 mthca_free_memfree(dev, qp);