Overview
This lecture explains race conditions in concurrent programming, demonstrates their causes, and examines software-only solutions—highlighting why reliable prevention ultimately demands hardware support for atomic operations.
Understanding Race Conditions
- A race condition occurs when multiple threads access and modify shared data concurrently, leading to unpredictable results.
- Even simple actions like
counter = counter + 1 are not atomic; they're broken into multiple CPU instructions.
- Non-atomic operations can be interrupted mid-way, causing threads to read outdated values and corrupt shared state.
- Concurrency means multiple programs share CPU time, creating the illusion of parallel execution.
- The critical section is the part of code where shared data is at risk of concurrent modification.
Identifying and Preventing Race Conditions
- Critical sections must be protected to avoid simultaneous access by multiple threads.
- Mutual exclusion ensures only one thread enters the critical section at a time.
- Naive solutions, such as using a shared flag variable to control access, often create new race conditions.
Peterson's Algorithm
- Uses two shared variables:
turn (whose turn it is) and a flag array (thread intent to enter).
- Each thread sets its flag, gives turn to the other, and spin-waits until the other is out.
- Peterson's Algorithm achieves mutual exclusion, progress, and bounded waiting in theory.
- Minor delays or outdated reads only cause extra spin-wait, not logic errors in the intended model.
Why Software-Only Solutions Fail
- Compilers may reorder instructions for optimization, breaking assumptions in synchronization code.
- CPUs also reorder instructions during execution for efficiency (CPU pipelining and out-of-order execution).
- Multi-core and multi-cache architectures can cause threads to see stale values in shared variables.
- These hardware-level behaviors make purely software-based mutual exclusion unreliable.
Reliable Solutions: Hardware Support
- Hardware-provided atomic operations are needed to ensure correct synchronization.
- Atomic operations prevent compiler or CPU reordering and guarantee visibility across cores.
- Purely software approaches like Peterson’s are not reliable on modern hardware.
Key Terms & Definitions
- Race Condition — bug where outcome depends on unpredictable ordering of thread execution.
- Atomic Operation — an indivisible operation that completes in a single step without interruption.
- Critical Section — code segment where shared data may be concurrently accessed or modified.
- Mutual Exclusion — property ensuring only one thread can enter the critical section at a time.
- Spin Wait (Spinlock) — thread repeatedly checks a condition, waiting for permission to enter a critical section.
- Peterson’s Algorithm — classic software-only algorithm for mutual exclusion using flags and a turn variable.
Action Items / Next Steps
- Expect future lectures to cover thread synchronization and atomic hardware instructions in detail.
- Review race condition concepts and be cautious about relying solely on software for mutual exclusion.