[$] A virtual filesystem locking surprise

It is well understood that concurrency makes programming problems harder;the high level of concurrency inherent in kernel development is one of thereasons why kernel work can be challenging. Things can get even worse,though, if concurrent access happens in places where the code is notexpecting it. The long story accompanying thisshort patch from Christian Brauner is illustrative of the kind ofproblem that can arise when assumptions about concurrency prove to beincorrect.