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Vol. 3 8-17
MULTIPLE-PROCESSOR MANAGEMENT
reader should note that reordering is prevented also if the locked instruction is
executed after a load or a store.
The first example illustrates that loads may not be reordered with earlier locked
instructions:
As explained in Section 8.2.3.8, there is a total order of the executions of locked
instructions. Without loss of generality, suppose that processor 0’s XCHG occurs first.
Because the Intel-64 memory-ordering model prevents processor 1’s load from
being reordered with its earlier XCHG, processor
0’s XCHG occurs before
processor 1’s load. This implies r4 == 1.
A similar argument (referring instead to processor 2’s accesses) applies if
processor 1’s XCHG occurs before processor 0’s XCHG.
The second example illustrates that a store may not be reordered with an earlier
locked instruction:
Assume r2 == 1.
• Because r2 == 1, processor 0’s store to y occurs before processor 1’s load from
y.
• Because the memory-ordering model prevents a store from being reordered with
an earlier locked instruction, processor
0’s XCHG into x occurs before its store to
y. Thus, processor 0’s XCHG into x occurs before processor 1’s load from y.
• Because the memory-ordering model prevents loads from being reordered (see
Section 8.2.3.2), processor 1’s loads occur in order and, therefore, processor 1’s
XCHG into x occurs before processor 1’s load from x. Thus, r3 == 1.
Example 8-9. Loads Are not Reordered with Locks
Processor 0 Processor 1
xchg [ _x], r1 xchg [ _y], r3
mov r2, [ _y] mov r4, [ _x]
Initially x == y == 0, r1 == r3 == 1
r2 == 0 and r4 == 0 is not allowed
Example 8-10. Stores Are not Reordered with Locks
Processor 0 Processor 1
xchg [ _x], r1 mov r2, [ _y]
mov [ _y], 1 mov r3, [ _x]
Initially x == y == 0, r1 == 1
r2 == 1 and r3 == 0 is not allowed