When mongod is under fsyncLock we will skip writing entries to the system.profile collection as they (and the associated operation) would be blocked on the fsyncLock global read lock. The check we perform for this however has no protection against fsyncLock state changing post-check but prior to lock acquisition for the profiler write.

https://github.com/mongodb/mongo/blob/r3.4.0-rc1/src/mongo/db/instance.cpp#L705