RDMA/mlx5: Reduce locking in implicit_mr_get_data()

Now that the child MRs are stored in an xarray we can rely on the SRCU
lock to protect the xa_load and use xa_cmpxchg on the slow allocation path
to resolve races with concurrent page fault.

This reduces the scope of the critical section of umem_mutex for implicit
MRs to only cover mlx5_ib_update_xlt, and avoids taking a lock at all if
the child MR is already in the xarray. This makes it consistent with the
normal ODP MR critical section for umem_lock, and the locking approach
used for destroying an unusued implicit child MR.

The MLX5_IB_UPD_XLT_ATOMIC is no longer needed in implicit_get_child_mr()
since it is no longer called with any locks.

Link: https://lore.kernel.org/r/20191009160934.3143-11-jgg@ziepe.ca
Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

diff --git a/drivers/infiniband/hw/mlx5/odp.c b/drivers/infiniband/hw/mlx5/odp.c
index 6f7eea175c72f22f7c6dc64a7cc5e770c504e59e..00e14b6acd98a4e6f354bae5d711714cce95cd01 100644 (file)
--- a/drivers/infiniband/hw/mlx5/odp.c
+++ b/drivers/infiniband/hw/mlx5/odp.c
@@ -381,8 +381,7 @@ static struct mlx5_ib_mr *implicit_get_child_mr(struct mlx5_ib_mr *imr,
                                 MLX5_IMR_MTT_ENTRIES,
                                 PAGE_SHIFT,
                                 MLX5_IB_UPD_XLT_ZAP |
-                                MLX5_IB_UPD_XLT_ENABLE |
-                                MLX5_IB_UPD_XLT_ATOMIC);
+                                MLX5_IB_UPD_XLT_ENABLE);
        if (err) {
                ret = ERR_PTR(err);
                goto out_release;
@@ -392,9 +391,16 @@ static struct mlx5_ib_mr *implicit_get_child_mr(struct mlx5_ib_mr *imr,
         * Once the store to either xarray completes any error unwind has to
         * use synchronize_srcu(). Avoid this with xa_reserve()
         */
-       err = xa_err(xa_store(&imr->implicit_children, idx, mr, GFP_KERNEL));
-       if (err) {
-               ret = ERR_PTR(err);
+       ret = xa_cmpxchg(&imr->implicit_children, idx, NULL, mr, GFP_KERNEL);
+       if (unlikely(ret)) {
+               if (xa_is_err(ret)) {
+                       ret = ERR_PTR(xa_err(ret));
+                       goto out_release;
+               }
+               /*
+                * Another thread beat us to creating the child mr, use
+                * theirs.
+                */
                goto out_release;
        }
 
@@ -424,7 +430,8 @@ static struct mlx5_ib_mr *implicit_mr_get_data(struct mlx5_ib_mr *imr,
        struct mlx5_ib_mr *result = NULL;
        int ret;
 
-       mutex_lock(&odp_imr->umem_mutex);
+       lockdep_assert_held(&imr->dev->odp_srcu);
+
        for (idx = idx; idx <= end_idx; idx++) {
                struct mlx5_ib_mr *mtt = xa_load(&imr->implicit_children, idx);
 
@@ -450,20 +457,27 @@ static struct mlx5_ib_mr *implicit_mr_get_data(struct mlx5_ib_mr *imr,
         */
 out:
        if (likely(!inv_len))
-               goto out_unlock;
+               return result;
 
+       /*
+        * Notice this is not strictly ordered right, the KSM is updated after
+        * the implicit_leaves is updated, so a parallel page fault could see
+        * a MR that is not yet visible in the KSM.  This is similar to a
+        * parallel page fault seeing a MR that is being concurrently removed
+        * from the KSM. Both of these improbable situations are resolved
+        * safely by resuming the HW and then taking another page fault. The
+        * next pagefault handler will see the new information.
+        */
+       mutex_lock(&odp_imr->umem_mutex);
        ret = mlx5_ib_update_xlt(imr, inv_start_idx, inv_len, 0,
                                 MLX5_IB_UPD_XLT_INDIRECT |
                                         MLX5_IB_UPD_XLT_ATOMIC);
+       mutex_unlock(&odp_imr->umem_mutex);
        if (ret) {
                mlx5_ib_err(to_mdev(imr->ibmr.pd->device),
                            "Failed to update PAS\n");
-               result = ERR_PTR(ret);
-               goto out_unlock;
+               return ERR_PTR(ret);
        }
-
-out_unlock:
-       mutex_unlock(&odp_imr->umem_mutex);
        return result;
 }