device not ready cuda

 1         cudaError_t err;
 2         cudaEvent_t start, stop;
 3         cudaEventCreate(&start);
 4         cudaEventCreate(&stop);
 5         err = cudaEventRecord(start, 0);
 6         f(err != cudaSuccess) {
 7           printf ("\n\n 1. Error: %s\n\n", cudaGetErrorString(err));
 8           exit(1);
 9         }
10         // actual code
11         cudaThreadSynchronize();
12         err = cudaEventRecord(stop, 0);
13         if(err != cudaSuccess) {
14           printf ("\n\n2. Error: %s\n\n", cudaGetErrorString(err));
15           exit(1);
16         }
17         err = cudaEventElapsedTime(&elapsed_time, start, stop);
18         f(err != cudaSuccess) {
19           printf ("\n\n 3. Error: %s\n\n", cudaGetErrorString(err));
20           exit(1);
21         }