17.11

Example: Out of Order Writes

THREADS AND LOCKS

The total set of actions may be pictured as follows:

to

 thread

main memory

fro

 thread

read

a

read

b

lock

 class

SynchSimple

load

a

load

b

assign

a

use

a

assign

b

use

b

store

a

store

b

printing

write

a

write

b

unlock

 class

SynchSimple

Here an arrow from action

A

 to action

B

 indicates that

A

 must precede

B

.

In what order may the actions by the main memory occur? Note that the rules

do not require that

write

a

 occur before

write

b

; neither do they require that

read

a

occur before

read

b

. Also, even though method

to

 is

synchronized

, method

fro

is not

synchronized

, so there is nothing to prevent the

read

 actions from occur 

ring between the

lock

 and

unlock

 actions. (The point is that declaring one method

synchronized

 does not of itself make that method behave as if it were atomic.)

As a result, the method

fro

 could still obtain either

1

 or

3

 for the value of

a

,

and independently could obtain either

2

 or

4

 for the value of

b

. In particular,

fro

might observe the value

1

 for

a

 and

4

 for

b

. Thus, even though

to

 does an

assign

to

a

 and then an

assign

 to

b

, the

write

 actions to main memory may be observed

by another thread to occur as if in the opposite order.

414