THREADS AND LOCKS

Execution Order

17.2

Thus the interaction of a thread with a variable over time consists of a

sequence of

use

,

assign

,

load

, and

store

 actions. Main memory performs a

read

action for every

load

 and a

write

 action for every

store

. A thread's interactions

with a lock over time consists of a sequence of

lock

 and

unlock

 actions. All the

globally visible behavior of a thread thus comprises all the thread's actions on

variables and locks.

17.2   Execution Order

The rules of execution order constrain the order in which certain events may

occur. There are four general constraints on the relationships among actions:

The actions performed by any one thread are totally ordered; that is, for any

two actions performed by a thread, one action precedes the other.

The actions performed by the main memory for any one variable are totally

ordered; that is, for any two actions performed by the main memory on the

same variable, one action precedes the other.

The actions performed by the main memory for any one lock are totally

ordered; that is, for any two actions performed by the main memory on the

same lock, one action precedes the other.

It is not permitted for an action to follow itself.

The last rule may seem trivial, but it does need to be stated separately and explic 

itly for completeness. Without it, it would be possible to propose a set of actions

by two or more threads and precedence relationships among the actions that

would satisfy all the other rules but would require an action to follow itself.

Threads do not interact directly; they communicate only through the shared

main memory. The relationships between the actions of a thread and the actions of

main memory are constrained in three ways:

Each

lock

 or

unlock

 action is performed jointly by some thread and the main

memory.

Each

load

 action by a thread is uniquely paired with a

read

 action by the main

memory such that the

load

 action follows the

read

 action.

Each

store

 action by a thread is uniquely paired with a

write

 action by the

main memory such that the

write

 action follows the

store

 action.

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