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Lock-based Concurrency Control (cont)

Consider the following schedule, using locks: 

T1(a): Lr(Y)     R(Y)           continued
T2(a):      Lr(X)    R(X) U(X)  continued

T1(b):      U(Y)         Lw(X) W(X) U(X)
T2(b): Lw(Y)....W(Y) U(Y)

(where Lr = read-lock, Lw = write-lock, U = unlock)

Locks correctly ensure controlled access to X and Y.

Despite this, the schedule is not serializable.  (Ex: prove this)