如何解释去pprof / mutex显示等待解锁？

I'm optimizing Go program performance. While looking at mutex profiling, I got

> go tool pprof -seconds 30 -ignore .*Shopify.*  http://HOST/debug/pprof/mutex
(pprof) top 20
Active filters:
ignore=.*Shopify.*
Showing nodes accounting for 3.08mins, 91.03% of 3.39mins total
Dropped 82 nodes (cum <= 0.02mins)
    flat  flat%   sum%        cum   cum%
3.08mins 91.03% 91.03%   3.08mins 91.03%  sync.(*Mutex).Unlock
       0     0% 91.03%   0.06mins  1.75%  ....func2
       0     0% 91.03%   0.06mins  1.75%  ....func3

Code snippet is

     .          .    502:    w.mu.Lock()

     .          .    ... some calculation

     .   5.02mins    510:    w.mu.Unlock()
     .          .    511:
     .          .    512:    return nil
     .          .    513:}

What I don't understand are:

• Why mutex profiling shows only top 1 has flat time, all the rest are 0?
• If it shows waiting on Lock then it probably means calculation takes too long, but what does it mean when it shows waiting on Unlock?