[automerger skipped] Speed up and slightly simplify Mutex
am: d29087ce98 -s ours
am skip reason: change_id Ie53c65f2ce774a8cb4d224e2c1b3a110eb880f0c with SHA1 81dc7ab1df is in history

Change-Id: I389b39f626fc3469b0a0931ea4a4bacd0fa52ba3