Locking : Mutex vs Spinlocks
Both mutexes and spinlocks are mechanisms used to protect shared data in concurrent programming, but they behave very differently in how a thread waits for access.
Mutex (Mutual Exclusion Lock) is kind of a key to use any resources in the system. If you have mutex, you can use the resource, if you don't wait till the mutex is released. Process goes to wait queue for that particular resource.
Spin lock is a mechanism in which the process needing a resource poll the lock on resource till it gets it. In other words, a spinlock is a lock where the thread continuously checks (spins) until the lock becomes available. It is also called as busy-waiting. Process will be busy in a loop till it gets the resource.
So we get the first difference there itself, the way both wait for the resource lock. In case of mutex, process goes in wait state and does not use processor while in spin-lock, process keeps on looping and hence uses the processor even while waiting.
When to use Mutex?
Use a mutex when:
• Critical section is long or unpredictable
• You want to save CPU usage
• You are in user-space applications
• Fair scheduling matters
Typical examples:
• File access
• Database operations
• Complex computations inside critical section
When to use Spinlock?
Use a spinlock when:
• Critical section is extremely short
• You are in low-level or real-time systems
• Context switching cost is too high
• You cannot afford sleeping (e.g., interrupt context)
Typical examples:
• Operating system kernels
• Device drivers
• High-performance concurrent data structures
Comparison of spin-lock and mutex
Spin-locks are best used when a piece of code cannot go to sleep state. Best example of such code would be interrupts request handlers.
Mutexes are best used in user space program where sleeping of process does not affect the system in catastrophic way.
Spin locks make sense when we have multi-processor systems or we have uni-processor system with preemption enabled, spin locks used in uni-processor systems without preemption enabled, may lead to deadlock where process goes on spinning forever.
Following table summarizes the above points.
| Criteria | Mutex | Spinlock |
| Mechanism |
Test for lock. If available, use the resource If not, go to wait queue |
Test for lock. If available, use the resource. If not, loop again and test the lock till you get the lock |
| When to use |
Used when putting process is not harmful like user space programs. Use when there will be considerable time before process gets the lock. |
Used when process should not be put to sleep like Interrupt service routines. Use when lock will be granted in reasonably short time. |
| Drawbacks | Incur process context switch and scheduling cost. | Processor is busy doing nothing till lock is granted, wasting CPU cycles. |
