SERVER-29794 added _comment as a new member of OperationContext, and exposed setComment()/getComment() to set/get the value of the new field.

Accessing _comment, however, is not thread-safe, and to ensure thread-safety:

• The client lock must be acquired before calling into setComment().
• The client lock must be acquired to getComment() from any thread that does not own the client associated with the operation (i.e., Client::getCurrent() != opCtx->getClient()).

The common practice is to use the client lock to serialize concurrent accesses to OperationContext fields that are not thread-safe. We currently do so to access _comment when preparing the currentOp output (see here).