Причина в том, что из журнала фиксации "_ = m.state" убедитесь, что m не равно nil.
commit 5bb3a66a973ea87494b9197091e8c1f122080627
Author: Rémy Oudompheng <oudomphe@phare.normalesup.org>
Date: Mon Apr 8 23:46:54 2013 +0200
sync, sync/atomic: do not corrupt race detector after a nil dereference.
The race detector uses a global lock to analyze atomic
operations. A panic in the middle of the code leaves the
lock acquired.
Similarly, the sync package may leave the race detectro
inconsistent when methods are called on nil pointers.
R=golang-dev, r, minux.ma, dvyukov, rsc, adg
CC=golang-dev
https://golang.org/cl/7981043