How to demonstrate Java instruction reordering problems?

This demonstrates reordering of certain assignments, out of 1M iterations there is usually couple of printed lines.

public class App {    public static void main(String[] args) {        for (int i = 0; i < 1000_000; i++) {            final State state = new State();            // a = 0, b = 0, c = 0            // Write values            new Thread(() -> {                state.a = 1;                // a = 1, b = 0, c = 0                state.b = 1;                // a = 1, b = 1, c = 0                state.c = state.a + 1;                // a = 1, b = 1, c = 2            }).start();            // Read values - this should never happen, right?            new Thread(() -> {                // copy in reverse order so if we see some invalid state we know this is caused by reordering and not by a race condition in reads/writes                // we don't know if the reordered statements are the writes or reads (we will se it is writes later)                int tmpC = state.c;                int tmpB = state.b;                int tmpA = state.a;                if (tmpB == 1 && tmpA == 0) {                    System.out.println("Hey wtf!! b == 1 && a == 0");                }                if (tmpC == 2 && tmpB == 0) {                    System.out.println("Hey wtf!! c == 2 && b == 0");                }                if (tmpC == 2 && tmpA == 0) {                    System.out.println("Hey wtf!! c == 2 && a == 0");                }            }).start();        }        System.out.println("done");    }    static class State {        int a = 0;        int b = 0;        int c = 0;    }}

Printing the assembly for the write lambda gets this output (among other..)

                                                ; {metadata('com/example/App$$Lambda$1')}  0x00007f73b51a0100: 752b                jne       7f73b51a012dh                                                ;*invokeinterface run                                                ; - java.lang.Thread::run@11 (line 748)  0x00007f73b51a0102: 458b530c            mov       r10d,dword ptr [r11+0ch]                                                ;*getfield arg$1                                                ; - com.example.App$$Lambda$1/1831932724::run@1                                                ; - java.lang.Thread::run@-1 (line 747)  0x00007f73b51a0106: 43c744d41402000000  mov       dword ptr [r12+r10*8+14h],2h                                                ;*putfield c                                                ; - com.example.App::lambda$main$0@17 (line 18)                                                ; - com.example.App$$Lambda$1/1831932724::run@4                                                ; - java.lang.Thread::run@-1 (line 747)                                                ; implicit exception: dispatches to 0x00007f73b51a01b5  0x00007f73b51a010f: 43c744d40c01000000  mov       dword ptr [r12+r10*8+0ch],1h                                                ;*putfield a                                                ; - com.example.App::lambda$main$0@2 (line 14)                                                ; - com.example.App$$Lambda$1/1831932724::run@4                                                ; - java.lang.Thread::run@-1 (line 747)  0x00007f73b51a0118: 43c744d41001000000  mov       dword ptr [r12+r10*8+10h],1h                                                ;*synchronization entry                                                ; - java.lang.Thread::run@-1 (line 747)  0x00007f73b51a0121: 4883c420            add       rsp,20h  0x00007f73b51a0125: 5d                  pop       rbp  0x00007f73b51a0126: 8505d41eb016        test      dword ptr [7f73cbca2000h],eax                                                ;   {poll_return}  0x00007f73b51a012c: c3                  ret  0x00007f73b51a012d: 4181f885f900f8      cmp       r8d,0f800f985h

I am not sure why the last mov dword ptr [r12+r10*8+10h],1h is not marked with putfield b and line 16, but you can see the swapped assignment of b and c (c right after a).

EDIT:Because writes happen in order a,b,c and reads happen in reverse order c,b,a you should never see an invalid state unless the writes (or reads) are reordered.

Writes performed by single cpu (or core) are visible in same order by all processors, see e.g. this answer, which points to Intel System Programming Guide Volume 3 section 8.2.2.

Writes by a single processor are observed in the same order by all processors.

Test

I wrote a JUnit 5 test that checks whether instruction reordering took place after two threads terminate.

• The test must pass if no instruction reordering happened.
• The test must fail if instruction reordering occurred.

‌

public class InstructionReorderingTest {    static int x, y, a, b;    @org.junit.jupiter.api.BeforeEach    public void init() {        x = y = a = b = 0;    }    @org.junit.jupiter.api.Test    public void test() throws InterruptedException {        Thread threadA = new Thread(() -> {            a = 1;            x = b;        });        Thread threadB = new Thread(() -> {            b = 1;            y = a;        });        threadA.start();        threadB.start();        threadA.join();        threadB.join();        org.junit.jupiter.api.Assertions.assertFalse(x == 0 && y == 0);    }}

Results

I ran the test until it fails several times. The results are as follows:

InstructionReorderingTest.test [*] (12s 222ms): 29144 total, 1 failed, 29143 passed.InstructionReorderingTest.test [*] (26s 678ms): 69513 total, 1 failed, 69512 passed.InstructionReorderingTest.test [*] (12s 161ms): 27878 total, 1 failed, 27877 passed.

Explanation

The results we expect are

• x = 0, y = 1: threadA runs to completion before threadB starts.
• x = 1, y = 0: threadB runs to completion before threadA starts.
• x = 1, y = 1: their instructions are interleaved.

No one can expect x = 0, y = 0, which may happen as the test results showed.

The actions in each thread have no dataflow dependence on each other, and accordingly can be executed out of order. (Even if they are executed in order, the timing by which caches are flushed to main memory can make it appear, from the perspective of threadB, that the assignments in threadA occurred in the opposite order.)

_{Java Concurrency in Practice, Brian Goetz}

For single thread executions reordering is not a problem at all, because of Java Memory Model (JMM) (guarantee that any reads actions related to writes are total ordered) and cannot lead to unexpected results.

For concurrent execution, rules are completely different and things gets more complicated to understand (even by providing simple example which will raise even more questions). But even this are totally described by JMM with all corner cases, so, unexpected results also forbidden. Generally, forbidden if all barriers are placed right.

For better understanding reordering I strongly recommend this subject with plenty examples insides.