RDMA/ucma: Do not use file->mut to lock destroying

The only reader of destroying is inside a handler under the handler_mutex,
so directly use the handler_mutex when setting it instead of the larger
file->mut.

As the refcount could be zero here, and the cm_id already freed, and
additional refcount grab around the locking is required to touch the
cm_id.

Link: https://lore.kernel.org/r/20200818120526.702120-8-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/ucma.c

@@ -602,9 +602,17 @@ static int ucma_free_ctx(struct ucma_context *ctx)
 
 static int __destroy_id(struct ucma_context *ctx)
 {
-	mutex_lock(&ctx->file->mut);
-	ctx->destroying = 1;
-	mutex_unlock(&ctx->file->mut);
+	/*
+	 * If the refcount is already 0 then ucma_close_id() has already
+	 * destroyed the cm_id, otherwise holding the refcount keeps cm_id
+	 * valid. Prevent queue_work() from being called.
+	 */
+	if (refcount_inc_not_zero(&ctx->ref)) {
+		rdma_lock_handler(ctx->cm_id);
+		ctx->destroying = 1;
+		rdma_unlock_handler(ctx->cm_id);
+		ucma_put_ctx(ctx);
+	}
 
 	flush_workqueue(ctx->file->close_wq);
 	/* At this point it's guaranteed that there is no inflight closing task */