易变的背负式.这足以可见性吗? [英] Volatile piggyback. Is this enough for visiblity?

问题描述

This is about volatile piggyback. Purpose: I want to reach a lightweight vars visibilty. Consistency of a_b_c is not important. I have a bunch of vars and I don't want to make them all volatile.

Is this code threadsafe?

class A {
    public int a, b, c;
    volatile int sync;

    public void setup() {
        a = 2;
        b = 3;
        c = 4;
    }

    public void sync() {
        sync++;
    }
}

final static A aaa = new A();

Thread0:
aaa.setup();
end

Thread1:
for(;;) {aaa.sync(); logic with aaa.a, aaa.b, aaa.c}

Thread2:
for(;;) {aaa.sync(); logic with aaa.a, aaa.b, aaa.c}

解决方案

Java Memory Model defines the happens-before relationship which has the following properties (amongst others):

• "Each action in a thread happens-before every action in that thread that comes later in the program order" (program order rule)
• "A write to a volatile field happens-before every subsequent read of that same volatile" (volatile variable rule)

These two properties together with transitivity of the happens-before relationship imply the visibility guarantees that OP seeks in the following manner:

1. A write to a in thread 1 happens-before a write to sync in a call to sync() in thread 1 (program order rule).
2. The write to sync in the call to sync() in thread 1 happens-before a read to sync in a call to sync in thread 2 (volatile variable rule).
3. The read from sync in the call to sync() in thread 2 happens-before a read from a in thread 2 (program order rule).

This implies that the answer to the question is yes, i.e. the call to sync() in each iteration in threads 1 and 2 ensures visibility of changes to a, b and c to the other thread(s). Note that this ensures visibility only. No mutual exclusion guarantees exist and hence all invariants binding a, b and c may be violated.

See also Java theory and practice: Fixing the Java Memory Model, Part 2. In particular the section "New guarantees for volatile" which says

Under the new memory model, when thread A writes to a volatile variable V, and thread B reads from V, any variable values that were visible to A at the time that V was written are guaranteed now to be visible to B.

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