Differences and Application Scenarios Between Spinlocks and Mutexes in Operating Systems

1. Problem Background: Why Do We Need Locks?

In multi-threaded or multi-process environments, multiple execution flows may simultaneously access shared resources (such as global variables, data structures, etc.). Without proper synchronization control over such access, data inconsistency issues can arise. For example, if two threads perform a "read-modify-write" operation on a counter simultaneously, one update might be lost. Locks are a fundamental synchronization mechanism to solve this problem, ensuring that only one execution flow can enter the critical section (the code segment accessing shared resources) at any given time.

2. Two Basic Locks: Spinlocks and Mutexes

Spinlocks and Mutexes are two of the most common lock implementations. Their core difference lies in the behavior of a waiting thread when the lock is already held.

2.1 How Spinlocks Work

• Core Idea: If a thread attempts to acquire a lock and finds it already held, it does not relinquish the CPU. Instead, it continuously checks the lock's status (i.e., "spins") in a tight loop until the lock is released.
• Behavior Description:
  1. Thread A attempts to acquire a spinlock. If the lock is free, A obtains it and enters the critical section.
  2. Thread B then attempts to acquire the same lock. Since the lock is held by A, B starts repeatedly checking in a tight loop to see if the lock becomes available.
  3. Thread A finishes executing the critical section code and releases the lock.
  4. Thread B detects that the lock is released, immediately acquires it, and enters the critical section.
• Key Point: The waiting thread (B) continuously occupies the CPU core during the wait, constantly executing the "check-wait" loop.

2.2 How Mutexes Work

• Core Idea: If a thread attempts to acquire a lock and finds it already held, the operating system blocks that thread and removes it from the CPU (placing it in a wait queue). When the lock is released, the OS wakes up one of the waiting threads.
• Behavior Description:
  1. Thread A attempts to acquire a mutex. If the lock is free, A obtains it and enters the critical section.
  2. Thread B attempts to acquire the same lock. Since the lock is held, the OS sets thread B's state to "blocked" and removes it from the run queue. B no longer consumes CPU time.
  3. Thread A releases the lock. The OS kernel detects that threads are waiting for this lock, changes thread B's state to "ready," and places it back into the run queue.
  4. At some point, the scheduler schedules thread B to run, and B successfully acquires the lock.
• Key Point: The waiting thread (B) does not consume CPU resources during the wait, but the thread switch (context switch) incurs a certain overhead.

3. Performance Comparison and Key Differences

The fundamental difference between them leads to different performance characteristics and suitable application scenarios.

4. In-depth Analysis: Context Switch Overhead

Understanding "context switch overhead" is key to grasping the difference between the two.

• What is a Context Switch? When the CPU switches from one thread to another, it needs to save the current thread's register state, program counter, etc., and load the corresponding state of the new thread. This process is performed by the OS kernel and consumes hundreds or even thousands of CPU clock cycles.
• Why Do Mutexes Have This Overhead? Because when a thread fails to acquire a lock, it actively invokes a system call to block itself, which inevitably triggers a switch from user mode to kernel mode and a thread schedule. When releasing the lock, another switch is needed to wake the waiting thread.

5. Application Scenario Analysis

The choice of lock depends on the characteristics of the critical section and the system environment.

5.1 When to Use Spinlocks?

• Critical section code is extremely short: If the execution time of the critical section is shorter than the overhead of a complete context switch, using a spinlock is more efficient. Because the cost of putting a thread to sleep and waking it up might be greater than letting it spin briefly.
• On multi-core processors: Spinlocks assume the lock holder is running on another core and will release the lock soon. On a single-core CPU, spinning is meaningless if the lock is held (because the thread holding the lock cannot run), unless used to disable kernel preemption.
• In contexts where sleeping is not allowed: For example, in kernel interrupt service routines (ISRs), scheduling (and thus sleeping) is not allowed, so spinlocks must be used.

5.2 When to Use Mutexes?

• Critical section code is long: If the critical section involves time-consuming operations like I/O or complex calculations, the wait time might be long. Using a mutex allows waiting threads to sleep immediately, yielding CPU resources to other useful threads, thereby improving overall system throughput.
• In user-mode applications: For most application development, mutexes are a more common and recommended choice because they do not waste CPU resources and avoid the risk of significant performance degradation if a thread holds the lock for a slightly longer time.

6. Summary and Analogy

• Spinlock: Like continuously knocking on a door asking, "Are you done yet?" Suitable for situations where you only need to wait a short while. If you wait too long, you get tired (wasting CPU) and hinder others (wasting system resources).
• Mutex: Like taking a number and waiting in line. You'll be called when it's your turn. Suitable for situations where the wait time is uncertain or long. While waiting, you can do other things (the CPU executes other threads).

In practical systems (like the Linux kernel), adaptive locks or other hybrid strategies are often employed: first attempting to spin for a short time, and if the lock is still not acquired, then switching to sleep-waiting, aiming to balance the two types of overhead.