diff --git a/models/contest.js b/models/contest.js
index e59eb01..8bcfda3 100644
--- a/models/contest.js
+++ b/models/contest.js
@@ -120,21 +120,20 @@ class Contest extends Model {
     let problems = await this.getProblems();
     if (!problems.includes(judge_state.problem_id)) throw new ErrorMessage('当前比赛中无此题目。');
 
-    await syzoj.utils.lock('Contest::newSubmission', 'create_player', judge_state.user_id);
-
-    let player = await ContestPlayer.findInContest({
-      contest_id: this.id,
-      user_id: judge_state.user_id
-    });
-
-    if (!player) {
-      player = await ContestPlayer.create({
+    let player;
+    await syzoj.utils.lock(['Contest::newSubmission', 'create_player', judge_state.user_id], async () => {
+      player = await ContestPlayer.findInContest({
         contest_id: this.id,
         user_id: judge_state.user_id
       });
-    }
 
-    syzoj.utils.unlock('Contest::newSubmission', 'create_player', judge_state.user_id);
+      if (!player) {
+        player = await ContestPlayer.create({
+          contest_id: this.id,
+          user_id: judge_state.user_id
+        });
+      }
+    });
 
     await player.updateScore(judge_state);
     await player.save();
diff --git a/models/contest_player.js b/models/contest_player.js
index c64659a..09480f9 100644
--- a/models/contest_player.js
+++ b/models/contest_player.js
@@ -69,100 +69,98 @@ class ContestPlayer extends Model {
   }
 
   async updateScore(judge_state) {
-    await syzoj.utils.lock('ContestPlayer::updateScore', this.id);
+    await syzoj.utils.lock(['ContestPlayer::updateScore', this.id], async () => {
+      await this.loadRelationships();
+      if (this.contest.type === 'ioi') {
+        if (!judge_state.pending) {
+          if (!this.score_details[judge_state.problem_id]) {
+            this.score_details[judge_state.problem_id] = {
+              score: judge_state.score,
+              judge_id: judge_state.id,
+              submissions: {}
+            };
+          }
 
-    await this.loadRelationships();
-    if (this.contest.type === 'ioi') {
-      if (!judge_state.pending) {
-        if (!this.score_details[judge_state.problem_id]) {
-          this.score_details[judge_state.problem_id] = {
-            score: judge_state.score,
+          this.score_details[judge_state.problem_id].submissions[judge_state.id] = {
             judge_id: judge_state.id,
-            submissions: {}
+            score: judge_state.score,
+            time: judge_state.submit_time
           };
-        }
-
-        this.score_details[judge_state.problem_id].submissions[judge_state.id] = {
-          judge_id: judge_state.id,
-          score: judge_state.score,
-          time: judge_state.submit_time
-        };
 
-        let arr = Object.values(this.score_details[judge_state.problem_id].submissions);
-        arr.sort((a, b) => a.time - b.time);
+          let arr = Object.values(this.score_details[judge_state.problem_id].submissions);
+          arr.sort((a, b) => a.time - b.time);
 
-        let maxScoreSubmission = null;
-        for (let x of arr) {
-          if (!maxScoreSubmission || x.score >= maxScoreSubmission.score && maxScoreSubmission.score < 100) {
-            maxScoreSubmission = x;
+          let maxScoreSubmission = null;
+          for (let x of arr) {
+            if (!maxScoreSubmission || x.score >= maxScoreSubmission.score && maxScoreSubmission.score < 100) {
+              maxScoreSubmission = x;
+            }
           }
-        }
 
-        this.score_details[judge_state.problem_id].judge_id = maxScoreSubmission.judge_id;
-        this.score_details[judge_state.problem_id].score = maxScoreSubmission.score;
-        this.score_details[judge_state.problem_id].time = maxScoreSubmission.time;
+          this.score_details[judge_state.problem_id].judge_id = maxScoreSubmission.judge_id;
+          this.score_details[judge_state.problem_id].score = maxScoreSubmission.score;
+          this.score_details[judge_state.problem_id].time = maxScoreSubmission.time;
 
+          this.score = 0;
+          for (let x in this.score_details) {
+            this.score += this.score_details[x].score;
+          }
+        }
+      } else if (this.contest.type === 'noi') {
+        this.score_details[judge_state.problem_id] = {
+          score: judge_state.score,
+          judge_id: judge_state.id
+        };
         this.score = 0;
         for (let x in this.score_details) {
           this.score += this.score_details[x].score;
         }
-      }
-    } else if (this.contest.type === 'noi') {
-      this.score_details[judge_state.problem_id] = {
-        score: judge_state.score,
-        judge_id: judge_state.id
-      };
-      this.score = 0;
-      for (let x in this.score_details) {
-        this.score += this.score_details[x].score;
-      }
-    } else if (this.contest.type === 'acm') {
-      if (!judge_state.pending) {
-        if (!this.score_details[judge_state.problem_id]) {
-          this.score_details[judge_state.problem_id] = {
-            accepted: false,
-            unacceptedCount: 0,
-            acceptedTime: 0,
-            judge_id: 0,
-            submissions: {}
-          };
-        }
+      } else if (this.contest.type === 'acm') {
+        if (!judge_state.pending) {
+          if (!this.score_details[judge_state.problem_id]) {
+            this.score_details[judge_state.problem_id] = {
+              accepted: false,
+              unacceptedCount: 0,
+              acceptedTime: 0,
+              judge_id: 0,
+              submissions: {}
+            };
+          }
 
-        this.score_details[judge_state.problem_id].submissions[judge_state.id] = {
-          judge_id: judge_state.id,
-          accepted: judge_state.status === 'Accepted',
-          compiled: judge_state.status !== 'Compile Error',
-          time: judge_state.submit_time
-        };
+          this.score_details[judge_state.problem_id].submissions[judge_state.id] = {
+            judge_id: judge_state.id,
+            accepted: judge_state.status === 'Accepted',
+            compiled: judge_state.status !== 'Compile Error',
+            time: judge_state.submit_time
+          };
 
-        let arr = Object.values(this.score_details[judge_state.problem_id].submissions);
-        arr.sort((a, b) => a.time - b.time);
-
-        this.score_details[judge_state.problem_id].unacceptedCount = 0;
-        this.score_details[judge_state.problem_id].judge_id = 0;
-        for (let x of arr) {
-          if (x.accepted) {
-            this.score_details[judge_state.problem_id].accepted = true;
-            this.score_details[judge_state.problem_id].acceptedTime = x.time;
-            this.score_details[judge_state.problem_id].judge_id = x.judge_id;
-            break;
-          } else if (x.compiled) {
-            this.score_details[judge_state.problem_id].unacceptedCount++;
+          let arr = Object.values(this.score_details[judge_state.problem_id].submissions);
+          arr.sort((a, b) => a.time - b.time);
+
+          this.score_details[judge_state.problem_id].unacceptedCount = 0;
+          this.score_details[judge_state.problem_id].judge_id = 0;
+          for (let x of arr) {
+            if (x.accepted) {
+              this.score_details[judge_state.problem_id].accepted = true;
+              this.score_details[judge_state.problem_id].acceptedTime = x.time;
+              this.score_details[judge_state.problem_id].judge_id = x.judge_id;
+              break;
+            } else if (x.compiled) {
+              this.score_details[judge_state.problem_id].unacceptedCount++;
+            }
           }
-        }
 
-        if (!this.score_details[judge_state.problem_id].accepted) {
-          this.score_details[judge_state.problem_id].judge_id = arr[arr.length - 1].judge_id;
-        }
+          if (!this.score_details[judge_state.problem_id].accepted) {
+            this.score_details[judge_state.problem_id].judge_id = arr[arr.length - 1].judge_id;
+          }
 
-        this.score = 0;
-        for (let x in this.score_details) {
-          if (this.score_details[x].accepted) this.score++;
+          this.score = 0;
+          for (let x in this.score_details) {
+            if (this.score_details[x].accepted) this.score++;
+          }
         }
       }
-    }
-
-    syzoj.utils.unlock('ContestPlayer::updateScore', this.id);
+    });
   }
 
   getModel() { return model; }
diff --git a/models/judge_state.js b/models/judge_state.js
index 28f6cc1..546123d 100644
--- a/models/judge_state.js
+++ b/models/judge_state.js
@@ -191,43 +191,41 @@ class JudgeState extends Model {
   }
 
   async rejudge() {
-    await syzoj.utils.lock('JudgeState::rejudge', this.id);
+    await syzoj.utils.lock(['JudgeState::rejudge', this.id], async () => {
+      await this.loadRelationships();
 
-    await this.loadRelationships();
-
-    let oldStatus = this.status;
+      let oldStatus = this.status;
 
-    this.status = 'Waiting';
-    this.score = 0;
-    this.total_time = 0;
-    this.max_memory = 0;
-    this.pending = true;
-    this.result = { status: "Waiting", total_time: 0, max_memory: 0, score: 0, case_num: 0, compiler_output: "", pending: true };
-    await this.save();
-
-    let WaitingJudge = syzoj.model('waiting_judge');
-    let waiting_judge = await WaitingJudge.create({
-      judge_id: this.id
-    });
+      this.status = 'Waiting';
+      this.score = 0;
+      this.total_time = 0;
+      this.max_memory = 0;
+      this.pending = true;
+      this.result = { status: "Waiting", total_time: 0, max_memory: 0, score: 0, case_num: 0, compiler_output: "", pending: true };
+      await this.save();
 
-    await waiting_judge.save();
+      let WaitingJudge = syzoj.model('waiting_judge');
+      let waiting_judge = await WaitingJudge.create({
+        judge_id: this.id
+      });
 
-    if (oldStatus === 'Accepted') {
-      await this.user.refreshSubmitInfo();
-      await this.user.save();
-    }
+      await waiting_judge.save();
 
-    if (this.type === 0 || this.type === 2) {
       if (oldStatus === 'Accepted') {
-        this.problem.ac_num--;
-        await this.problem.save();
+        await this.user.refreshSubmitInfo();
+        await this.user.save();
       }
-    } else if (this.type === 1) {
-      let contest = await Contest.fromID(this.type_info);
-      await contest.newSubmission(this);
-    }
 
-    syzoj.utils.unlock('JudgeState::rejudge', this.id);
+      if (this.type === 0 || this.type === 2) {
+        if (oldStatus === 'Accepted') {
+          this.problem.ac_num--;
+          await this.problem.save();
+        }
+      } else if (this.type === 1) {
+        let contest = await Contest.fromID(this.type_info);
+        await contest.newSubmission(this);
+      }
+    });
   }
 
   getModel() { return model; }
diff --git a/models/user.js b/models/user.js
index 537900d..8d9926a 100644
--- a/models/user.js
+++ b/models/user.js
@@ -88,34 +88,32 @@ class User extends Model {
   }
 
   async refreshSubmitInfo() {
-    await syzoj.utils.lock('User::refreshSubmitInfo', this.id);
+    await syzoj.utils.lock(['User::refreshSubmitInfo', this.id], async () => {
+      let JudgeState = syzoj.model('judge_state');
+      let all = await JudgeState.model.findAll({
+        attributes: ['problem_id'],
+        where: {
+          user_id: this.id,
+          status: 'Accepted',
+          type: {
+            $ne: 1 // Not a contest submission
+          }
+        }
+      });
 
-    let JudgeState = syzoj.model('judge_state');
-    let all = await JudgeState.model.findAll({
-      attributes: ['problem_id'],
-      where: {
+      let s = new Set();
+      all.forEach(x => s.add(parseInt(x.get('problem_id'))));
+      this.ac_num = s.size;
+
+      let cnt = await JudgeState.count({
         user_id: this.id,
-        status: 'Accepted',
         type: {
           $ne: 1 // Not a contest submission
         }
-      }
-    });
-
-    let s = new Set();
-    all.forEach(x => s.add(parseInt(x.get('problem_id'))));
-    this.ac_num = s.size;
+      });
 
-    let cnt = await JudgeState.count({
-      user_id: this.id,
-      type: {
-        $ne: 1 // Not a contest submission
-      }
+      this.submit_num = cnt;
     });
-
-    this.submit_num = cnt;
-
-    syzoj.utils.unlock('User::refreshSubmitInfo', this.id);
   }
 
   async getACProblems() {
diff --git a/modules/api.js b/modules/api.js
index 4bf4643..186f074 100644
--- a/modules/api.js
+++ b/modules/api.js
@@ -95,35 +95,36 @@ app.get('/api/waiting_judge', async (req, res) => {
   try {
     if (req.query.session_id !== syzoj.config.judge_token) return res.status(404).send({ err: 'Permission denied' });
 
-    await syzoj.utils.lock('/api/waiting_judge');
+    let judge_state;
+    await syzoj.utils.lock('/api/waiting_judge', async () => {
+      let waiting_judge = await WaitingJudge.findOne();
+      if (!waiting_judge) {
+        return;
+      }
+
+      judge_state = await waiting_judge.getJudgeState();
+      await judge_state.loadRelationships();
+      await judge_state.problem.loadRelationships();
+      await waiting_judge.destroy();
+    });
 
-    let waiting_judge = await WaitingJudge.findOne();
-    if (!waiting_judge) {
-      syzoj.utils.unlock('/api/waiting_judge');
-      return res.send({ have_task: 0 });
+    if (judge_state) {
+      res.send({
+        have_task: 1,
+        judge_id: judge_state.id,
+        code: judge_state.code,
+        language: judge_state.language,
+        testdata: judge_state.problem.testdata ? judge_state.problem.testdata.md5 : '',
+        time_limit: judge_state.problem.time_limit,
+        memory_limit: judge_state.problem.memory_limit,
+        file_io: judge_state.problem.file_io,
+        file_io_input_name: judge_state.problem.file_io_input_name,
+        file_io_output_name: judge_state.problem.file_io_output_name
+      });
+    } else {
+      res.send({ have_task: 0 });
     }
-
-    let judge_state = await waiting_judge.getJudgeState();
-    await judge_state.loadRelationships();
-    await judge_state.problem.loadRelationships();
-    await waiting_judge.destroy();
-
-    syzoj.utils.unlock('/api/waiting_judge');
-
-    res.send({
-      have_task: 1,
-      judge_id: judge_state.id,
-      code: judge_state.code,
-      language: judge_state.language,
-      testdata: judge_state.problem.testdata ? judge_state.problem.testdata.md5 : '',
-      time_limit: judge_state.problem.time_limit,
-      memory_limit: judge_state.problem.memory_limit,
-      file_io: judge_state.problem.file_io,
-      file_io_input_name: judge_state.problem.file_io_input_name,
-      file_io_output_name: judge_state.problem.file_io_output_name
-    });
   } catch (e) {
-    syzoj.log(e);
     res.status(500).send(e);
   }
 });
diff --git a/utility.js b/utility.js
index e0fde00..bf987ab 100644
--- a/utility.js
+++ b/utility.js
@@ -74,30 +74,6 @@ function highlightPygmentize(code, lang, cb) {
 
 renderer.config.highlight = highlightPygmentize;
 
-let locks = {};
-let lock = function () {
-  let s = JSON.stringify(Array.from(arguments));
-  return new Promise((resolve, reject) => {
-    if (!locks[s]) {
-      locks[s] = {
-        lock: new AsyncLock(),
-        done: null
-      };
-    }
-
-    locks[s].lock.acquire('', done => {
-      locks[s].done = done;
-      resolve();
-    });
-  });
-};
-
-let unlock = function () {
-  let s = JSON.stringify(Array.from(arguments));
-  locks[s].done();
-  locks[s].done = null;
-}
-
 module.exports = {
   resolvePath(s) {
     let a = Array.from(arguments);
@@ -346,6 +322,10 @@ module.exports = {
   isValidUsername(s) {
     return /^[a-zA-Z0-9\-\_]+$/.test(s);
   },
-  lock: lock,
-  unlock: unlock
+  locks: [],
+  lock(key, cb) {
+    let s = JSON.stringify(key);
+    if (!this.locks[s]) this.locks[s] = new AsyncLock();
+    return this.locks[s].acquire(s, cb);
+  }
 };