On Sat, 2015-03-14 at 18:03 -0700, Davidlohr Bueso wrote:

Good analysis, but not quite accurate for one simple fact: mutex
trylocks _only_ use fastpaths (obviously just depend on the counter
cmpxchg to 0), so you never fallback to the slowpath you are mentioning,
thus the race is non existent. Please see the arch code.

For debug we use the trylock slowpath, but so does everything else, so
again you cannot hit this scenario.