17.5

Rules about Locks

THREADS AND LOCKS

be separated in time, with other actions coming between them. Consequently, if

two threads concurrently assign distinct values to the same shared non 

volatile

double

 or

long

 variable, a subsequent use of that variable may obtain a value that

is not equal to either of the assigned values, but some implementation dependent

mixture of the two values.

An implementation is free to implement

load

,

store

,

read

, and

write

 actions

for

double

 and

long

 values as atomic 64 bit actions; in fact, this is strongly

encouraged. The model divides them into 32 bit halves for the sake of several cur 

rently popular microprocessors that fail to provide efficient atomic memory trans 

actions on 64 bit quantities. It would have been simpler for Java to define all

memory transactions on single variables as atomic; this more complex definition

is a pragmatic concession to current hardware practice. In the future this conces 

sion may be eliminated. Meanwhile, programmers are cautioned always to explic 

itly synchronize access to shared

double

 and

long

 variables.

17.5   Rules about Locks

By the pricking of my thumbs,

Something wicked this way comes.

Open, locks,

Whoever knocks!

 William Shakespeare,

 Macbeth

, Act IV, scene i

Let

T

 be a thread and

L

 be a lock. There are certain constraints on the actions per 

formed by

T

 with respect to

L

:

A

lock

 action by

T

 on

L

 may occur only if, for every thread

S

 other than

T

, the

number of preceding

unlock

 actions by

S

 on

L

 equals the number of preceding

lock

 actions by

S

 on

L

. (Less formally: only one thread at a time is permitted

to lay claim to a lock, and moreover a thread may acquire the same lock mul 

tiple times and doesn't relinquish ownership of it until a matching number of

unlock

 actions have been performed.)

An

unlock

 action by thread

T

 on lock

L

 may occur only if the number of pre 

ceding

unlock

 actions by

T

 on

L

 is strictly less than the number of preceding

lock

 actions by

T

 on

L

. (Less formally: a thread is not permitted to unlock a

lock it doesn't own.)

With respect to a lock, the

lock

 and

unlock

 actions performed by all the

threads are performed in some total sequential order. This total order must be con 

sistent with the total order on the actions of each thread.

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