If your database allows you to set the isolation level as serializable, and supports MVCC, you can gain all of the safety you would need for a full banking application without getting any of the locking problems.
If the update that updates the count was still running, as long as you were careful to have the actual update of the comment in the same transaction, it would always be in sync, the comment just would not appear until the count had been updated.
A better question, however (Cobb would be proud) is why in the world are you storing the count of the replies on the post record??? The last activity and count are easily calculated using aggregate SQL functions.
There are, in effect, two issues.
1) Your database either cannot support concurrent use well, or it is not configured correctly.
2) You schema design looks to have some issues.