17.10

Example: Possible Swap

THREADS AND LOCKS

The total set of actions may be pictured as follows:

hither

 thread

main memory

yon

 thread

lock

 class

SynchSample

lock

 class

SynchSample

read

b

read

a

load

b

load

a

use

b

use

a

assign

a

assign

b

store

a

store

b

write

a

write

b

unlock

 class

SynchSample

unlock

 class

SynchSample

The

lock

 and

unlock

 actions provide further constraints on the order of actions

by the main memory; the

lock

 action by one thread cannot occur between the

lock

and

unlock

 actions of the other thread. Moreover, the

unlock

 actions require that

the

store

 and

write

 actions occur. It follows that only two sequences are possible:

write

a

read

a

,

read

b

write

b

 (then

ha=2

,

hb=2

,

ma=2

,

mb=2

,

ya=2

,

yb=2

)

read

a

write

a

,

write

b

read

b

 (then

ha=1

,

hb=1

,

ma=1

,

mb=1

,

ya=1

,

yb=1

)

While the resulting state is timing dependent, it can be seen that the two threads

will necessarily agree on the values of

a

 and

b

.

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