Data Race with Race Condition? Difference and Solution for both

2022-08-21

2 minute read

ios

data-race , race-condition , common

What are they? Matters? 🤔🆘

Data Race and Race Condition are DIFFERENT

Data-Race → MORE than 1 thread access to shared-resource (can be a variable or object) and AT LEAST 1 thread tries to modify that resource
Race-Condition → Order of threads’ execution leads to wrong program behavior
Many race conditions are due to data races, and many data races lead to race conditions. Can have race conditions without data races and data races without race conditions.

Example:

1@discardableResult
2func transfer(amount: Int, from source: BankAccount, to destination: BankAccount) -> Bool {
3   if source.balance >= amount {
4       destination.balance += amount
5       source.balance -= amount
6       return true
7   }
8   return false
9}

🔵 LET’S RUN ON ONE-SINGLE THREAD

1let accountA = BankAccount(balance: 100)
2let accountB = BankAccount(balance: 100)
3
4transfer(amount: 50, from: accountA, to: accountB)
5
6print(accountA.balance) // 5️⃣0️⃣
7print(accountB.balance) // 1️⃣5️⃣0️⃣️

It makes sense! ✅

🔴 LET’S RUN ON MULTIPLE THREADS

 1let accountA = BankAccount(balance: 1000)
 2let accountB = BankAccount(balance: 2000)
 3
 4...
 5@IBAction private func transferButtonTapped() {
 6    let group = DispatchGroup()
 7    for _ in 1...5 {
 8        DispatchQueue.global().async(group: group) {
 9            self.transfer(amount: 50, from: self.accountA, to: self.accountB)
10        }
11    }
12    
13    group.notify(queue: DispatchQueue.main) {
14        self.labelAccountA.text = "\(self.accountA.balance)"
15        self.labelAccountB.text = "\(self.accountB.balance)"
16    }
17}
18...

We will get a warning ⛔️ in transfer(amount:from:to) with the help of Thread Sanitizer as Data race in *.ViewController.transfer(amount: Swift.Int, from: BankAccount, to: BankAccount) -> Swift.Bool at 0x7b080002f8e0

ONLY mention about the ORDER of execution, which thread goes first will lead to different outcomes
IN REALITY, while a thread is checking the condition and execute destination.balance += amount (we suppose), other threads is also going through these steps and unexpectedly modify the same data → the outcome is unpredictable and can even cause crash 😫

Solutions

GCD to solve Data-Race

Using a serial queue, guarantee only one thread access balances Data-race is now addressed! ⚠️ Race-condition is still able to happen, cannot manage the order of execution of threads. But it won’t break the app, which is acceptable!

 1let serialQueue = DispatchQueue(label: "serial.queue")
 2
 3@discardableResult
 4func transfer(amount: Int, from source: BankAccount, to destination: BankAccount) -> Bool {
 5    serialQueue.sync {
 6        if source.balance >= amount {
 7            destination.balance += amount
 8            source.balance -= amount
 9            return
10        }
11        
12        return false
13    }
14}

📚 Learned from: