How to convert Mat <-> UMat back-and-forth

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I'd like to know how to convert UMat image onto Mat and back-and-forth. I do calculation on input image which I read/write

UMat img,gray; 
imread("lena.jpg", 1).copyTo(img); 
cvtColor(img, gray, COLOR_BGR2GRAY);  
imwrite("lena_gray.jpg", gray);

I have OpenCV 3.4.1. and after imwrite() call I got

Segmentation fault (core dumped)

There is a bug #11176 of segmentation fault if UMat saved by imwrite() call So I'm thinking to convert to Mat and after then to save. How to convert UMat to Mat back-and-forth? Or is there any other solution to write UMat image instead?