How to get a row level share lock

Question

Is it possible to acquire a row-level share lock, which would do all of the following?
1. Prevent others from updating that row.
2. Allow others to read that row.
3. Allow others to update other rows in the same table.
I have the following scenario where two transactions need to lock eachother out:
Set-up:
Insert into TABLE_A(value_a) values ('ok');
Insert into TABLE_B(value_b) values ('ok');
Transaction A:
Select value_b from TABLE_B
If value_b = 'ok', update TABLE_A set value_a = 'not ok'
Transaction B:
Select value_a from TABLE_A
If value_a = 'ok', update TABLE_B set value_b = 'not ok'
If transaction A runs first then the end result is "not ok" only in TABLE_A.
If transaction B runs first then the end result is "not ok" only in TABLE_B.
If the two transactions run concurrently, it is possible to get "not ok" in both tables. This is what I would like to prevent.
One way to get what I want is to use "select for update":
Transaction A:
Select value_a from TABLE_A for update
Select value_b from TABLE_B for update
If value_b = 'ok', update TABLE_A set value_a = 'not ok'
Transaction B:
Select value_a from TABLE_A for update
Select value_b from TABLE_B for update
If value_b = 'ok', update TABLE_B set value_a = 'not ok'
This way both transactions won't perform their update unless they know that the result if their select will still be the same after they commit. However, by using "select for update" Transaction A has gained an exclusive lock on the TABLE_B row. If a Transaction C with the same contents as Transaction A happens concurrently, then the two will block eachother even though all they both want is to read data from the same table.
Another way is to use "lock table", however using that would block out not only writes to a specific row, but writes to all rows in the table. (In my example there is only one row, but obviously that's just a simplified example.)
I have looked at the "serializable" isolation level, but that doesn't seem to help because the queries and updates involve more than one table.
I know that "reads don't block writes" is a fundamental part of the Oracle design that makes Oracle what it is, but is there any way I can explicitly make it happen anyway? Or can anyone see some other solution to what I'm trying to achieve?

Answer 1

Let me give a more real-world example to show what I'm talking about.
Imagine a simple bug-reporting OLTP application. Each bug has 3 fields:
1. Description
2. Resolution
3. Status (open/closed)
The application also has some rules:
A. If the Status is "closed", Description and Resolution cannot be changed.
B. In order to change the Status to "closed", both a Description and Resolution must be specified.
Now as long as only one person at a time updates a bug, everything is fine. However, if one person switches the Status to "closed" while another concurrently blanks out the Resolution you end up with a closed bug that has no Resolution. This is why some locking is necessary.
If Description, Resolution, and Status are all stored in the same table row, then it makes sense for each transaction to simply select the whole row FOR UPDATE, and then UPDATE it after it ensures all the rules are passed. However, in my fictional bug-reporting application they are all stored in separate tables. Now, the transaction that is making the change to Resolution could lock the row in the Status table FOR UPDATE (as everyone so far has suggested). Transactions making a similar change to the Description should then also lock the row in the Status table FOR UPDATE for the same reason. The end result of this is that Description changes and Resolution changes lock eachother out (can't happen concurrently, are serialized) when they don't need to be. Description changes and Resolution changes don't affect eachother, but because they both need to (exclusively) lock the same row, they are serialized.
If there was a way to acquire a share lock on the row in the Status table, then Description and Resolution changes would use that and not affect eachother. Status changes would still acquire an exclusive lock on the row, and would be blocked by (and block any further) concurrent Description and Resolution changes. This would be ideal.