diff --git a/cli/tests/unit/flock_test.ts b/cli/tests/unit/flock_test.ts
index 13d09bcf56..6c4f1e90d0 100644
--- a/cli/tests/unit/flock_test.ts
+++ b/cli/tests/unit/flock_test.ts
@@ -1,102 +1,164 @@
 // Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
 import { assertEquals, unitTest } from "./test_util.ts";
+import { readAll } from "../../../test_util/std/io/util.ts";
 
 unitTest(
   { perms: { read: true, run: true, hrtime: true } },
   async function flockFileSync() {
-    const path = "cli/tests/testdata/fixture.json";
-    const script = (exclusive: boolean, wait: number) => `
-          const { rid } = Deno.openSync("${path}");
-          Deno.flockSync(rid, ${exclusive ? "true" : "false"});
-          await new Promise(res => setTimeout(res, ${wait}));
-          Deno.funlockSync(rid);
-      `;
-    const run = (e: boolean, w: number) =>
-      Deno.run({ cmd: [Deno.execPath(), "eval", "--unstable", script(e, w)] });
-    const firstBlocksSecond = async (
-      first: boolean,
-      second: boolean,
-    ): Promise<boolean> => {
-      const firstPs = run(first, 1000);
-      await new Promise((res) => setTimeout(res, 250));
-      const start = performance.now();
-      const secondPs = run(second, 0);
-      await secondPs.status();
-      const didBlock = (performance.now() - start) > 500;
-      firstPs.close();
-      secondPs.close();
-      return didBlock;
-    };
-
-    assertEquals(
-      await firstBlocksSecond(true, false),
-      true,
-      "exclusive blocks shared",
-    );
-    assertEquals(
-      await firstBlocksSecond(false, true),
-      true,
-      "shared blocks exclusive",
-    );
-    assertEquals(
-      await firstBlocksSecond(true, true),
-      true,
-      "exclusive blocks exclusive",
-    );
-    assertEquals(
-      await firstBlocksSecond(false, false),
-      false,
-      "shared does not block shared",
-    );
+    await runFlockTests({ sync: true });
   },
 );
 
 unitTest(
   { perms: { read: true, run: true, hrtime: true } },
   async function flockFileAsync() {
-    const path = "cli/tests/testdata/fixture.json";
-    const script = (exclusive: boolean, wait: number) => `
-          const { rid } = await Deno.open("${path}");
-          await Deno.flock(rid, ${exclusive ? "true" : "false"});
-          await new Promise(res => setTimeout(res, ${wait}));
-          await Deno.funlock(rid);
-      `;
-    const run = (e: boolean, w: number) =>
-      Deno.run({ cmd: [Deno.execPath(), "eval", "--unstable", script(e, w)] });
-    const firstBlocksSecond = async (
-      first: boolean,
-      second: boolean,
-    ): Promise<boolean> => {
-      const firstPs = run(first, 1000);
-      await new Promise((res) => setTimeout(res, 250));
-      const start = performance.now();
-      const secondPs = run(second, 0);
-      await secondPs.status();
-      const didBlock = (performance.now() - start) > 500;
-      firstPs.close();
-      secondPs.close();
-      return didBlock;
-    };
-
-    assertEquals(
-      await firstBlocksSecond(true, false),
-      true,
-      "exclusive blocks shared",
-    );
-    assertEquals(
-      await firstBlocksSecond(false, true),
-      true,
-      "shared blocks exclusive",
-    );
-    assertEquals(
-      await firstBlocksSecond(true, true),
-      true,
-      "exclusive blocks exclusive",
-    );
-    assertEquals(
-      await firstBlocksSecond(false, false),
-      false,
-      "shared does not block shared",
-    );
+    await runFlockTests({ sync: false });
   },
 );
+
+async function runFlockTests(opts: { sync: boolean }) {
+  assertEquals(
+    await checkFirstBlocksSecond({
+      firstExclusive: true,
+      secondExclusive: false,
+      sync: opts.sync,
+    }),
+    true,
+    "exclusive blocks shared",
+  );
+  assertEquals(
+    await checkFirstBlocksSecond({
+      firstExclusive: false,
+      secondExclusive: true,
+      sync: opts.sync,
+    }),
+    true,
+    "shared blocks exclusive",
+  );
+  assertEquals(
+    await checkFirstBlocksSecond({
+      firstExclusive: true,
+      secondExclusive: true,
+      sync: opts.sync,
+    }),
+    true,
+    "exclusive blocks exclusive",
+  );
+  assertEquals(
+    await checkFirstBlocksSecond({
+      firstExclusive: false,
+      secondExclusive: false,
+      sync: opts.sync,
+    }),
+    false,
+    "shared does not block shared",
+  );
+}
+
+async function checkFirstBlocksSecond(opts: {
+  firstExclusive: boolean;
+  secondExclusive: boolean;
+  sync: boolean;
+}) {
+  const firstProcess = runFlockTestProcess({
+    exclusive: opts.firstExclusive,
+    sync: opts.sync,
+  });
+  const secondProcess = runFlockTestProcess({
+    exclusive: opts.secondExclusive,
+    sync: opts.sync,
+  });
+  try {
+    const sleep = (time: number) => new Promise((r) => setTimeout(r, time));
+
+    // wait for both processes to signal that they're ready
+    await Promise.all([firstProcess.waitSignal(), secondProcess.waitSignal()]);
+
+    // signal to the first process to enter the lock
+    await firstProcess.signal();
+    await firstProcess.waitSignal(); // entering signal
+    await firstProcess.waitSignal(); // entered signal
+    await sleep(20);
+    // signal the second to enter the lock
+    await secondProcess.signal();
+    await secondProcess.waitSignal(); // entering signal
+    await sleep(20);
+    // signal to the first to exit the lock
+    await firstProcess.signal();
+    await sleep(20);
+    // signal to the second to exit the lock
+    await secondProcess.waitSignal(); // entered signal
+    await secondProcess.signal();
+    // collect the remaining JSON output of both processes
+    const firstPsTimes = await firstProcess.getTimes();
+    const secondPsTimes = await secondProcess.getTimes();
+    return firstPsTimes.exitTime < secondPsTimes.enterTime;
+  } finally {
+    firstProcess.close();
+    secondProcess.close();
+  }
+}
+
+function runFlockTestProcess(opts: { exclusive: boolean; sync: boolean }) {
+  const path = "cli/tests/testdata/fixture.json";
+  const scriptText = `
+    const { rid } = Deno.openSync("${path}");
+
+    // ready signal
+    Deno.stdout.writeSync(new Uint8Array(1));
+    // wait for enter lock signal
+    Deno.stdin.readSync(new Uint8Array(1));
+
+    // entering signal
+    Deno.stdout.writeSync(new Uint8Array(1));
+    // lock and record the entry time
+    ${
+    opts.sync
+      ? `Deno.flockSync(rid, ${opts.exclusive ? "true" : "false"});`
+      : `await Deno.flock(rid, ${opts.exclusive ? "true" : "false"});`
+  }
+    const enterTime = new Date().getTime();
+    // entered signal
+    Deno.stdout.writeSync(new Uint8Array(1));
+
+    // wait for exit lock signal
+    Deno.stdin.readSync(new Uint8Array(1));
+
+    // record the exit time and wait a little bit before releasing
+    // the lock so that the enter time of the next process doesn't
+    // occur at the same time as this exit time (do double the
+    // windows clock resolution)
+    const exitTime = new Date().getTime();
+    await new Promise((resolve) => setTimeout(resolve, 30));
+
+    // release the lock
+    ${opts.sync ? "Deno.funlockSync(rid);" : "await Deno.funlock(rid);"}
+
+    // output the enter and exit time
+    console.log(JSON.stringify({ enterTime, exitTime }));
+`;
+
+  const process = Deno.run({
+    cmd: [Deno.execPath(), "eval", "--unstable", scriptText],
+    stdout: "piped",
+    stdin: "piped",
+  });
+
+  return {
+    waitSignal: () => process.stdout.read(new Uint8Array(1)),
+    signal: () => process.stdin.write(new Uint8Array(1)),
+    getTimes: async () => {
+      const outputBytes = await readAll(process.stdout);
+      const text = new TextDecoder().decode(outputBytes);
+      return JSON.parse(text) as {
+        enterTime: number;
+        exitTime: number;
+      };
+    },
+    close: () => {
+      process.stdout.close();
+      process.stdin.close();
+      process.close();
+    },
+  };
+}