Exercise 3.43.  Suppose that the balances in three accounts start out as 
$10, $20, and $30, and that multiple processes run, exchanging the 
balances in the accounts. Argue that if the processes are run 
sequentially, after any number of concurrent exchanges, the account 
balances should be $10, $20, and $30 in some order. Draw a timing 
diagram like the one in figure 3.29  to show how this condition can be 
violated if the exchanges are implemented using the first version of the 
account-exchange program in this section. On the other hand, argue that 
even with this exchange  program, the sum of the balances in the 
accounts will be preserved. Draw a timing diagram to show how even this 
condition would be violated if we did not serialize the transactions on 
individual accounts.

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Each exchange between any two bank accounts, given that the balances 
before the exchange are, $10, $20, and $30, will only modify either 
account inside a procedure which is serialized on both accounts.  This 
ensures that no exchange procedure will read or write to the accounts 
while another is running.  Since after each exchange, the account 
balances are still $10, $20, and $30 in some order, and exchanges are 
fully sequenced, any number of exchanges will leave the balances at 
these values.

If the first procedure is used as described in the text, on three 
accounts a1, a2, and a3, it is possible for the following to happen:

starting balances:
a1: $10
a2: $20
a3: $30

Peter computes a difference between a1 and a2 of $10.
Paul then exchanges a1 with a3, so the balance of a1 is now $30.
Peter's procedure then completes, transferring $10 from a2 to a1.

The closing balances are $40 for a1, and $10 each in the other 
two accounts.

The total balance of all three accounts is preserved, even though 
there is no sequence of exchanges that would give this result.

"Draw a timing diagram to show how even this condition would be violated 
if we did not serialize the transactions on individual accounts."

This is too similar to earlier exercises; skipped.