Have you ever wondered why your computer or phone suddenly froze? The culprit could be a nasty condition called deadlock that can bring even the most powerful systems to a grinding halt. In this guide, I‘ll walk you through everything you need to know about deadlocks – from what causes them to real-world failures they‘ve created.
Understanding Deadlocks
Deadlocks have been an enduring challenge in computing since the era of mainframe computers in the 1950s. They occur when two or more processes become intertwined, each waiting on the other to release resources needed to continue execution. This results in a standstill that crashes programs, operating systems, and everything relying on the halted processes.
Over the decades, many technologies from enterprise databases to the Mars Pathfinder mission have succumbed to catastrophic system failures rooted in deadlock conditions. So understanding deadlocks remains as relevant as ever for modern system reliability and stability.
Necessary Conditions
There are four fundamental conditions that must be met for a deadlock to materialize, known as Coffman conditions after their 1970s discoverer Edward G. Coffman Jr:
1. Mutual Exclusion
At least one resource must be non-sharable amongst processes. For example, a single-threaded CPU core or a printer can only handle one process at a time. These types of scarce resources force processes to compete.
2. Hold and Wait
Processes must be able to hold allocated resources while awaiting additional resources held by other processes. For example, consider two processes needing access to a printer and a CPU core to complete tasks.
3. No Preemption
Resources like locks can only be released explicitly by a process when it is finished. They cannot be forcibly revoked from blocked processes or threads.
4. Circular Wait
A circular chain exists where each process holds one resource that the next process in the chain requires. This circular interdependency creates a gridlock.
| Process | Holds | Waiting For |
|---|---|---|
| P1 | Printer | CPU |
| P2 | CPU | Printer |
Table 1: Circular wait example
Deadlocks will arise if all four conditions exist concurrently. Eliminating any single condition is sufficient to prevent deadlocks from occurring in theory. However, in complex real-world computer systems dynamically avoiding all four conditions is extremely difficult.
Next we‘ll explore approaches actually used in practice to manage deadlocks.
Deadlock Management Strategies
Since fully preventing deadlocks is often impractical, most operating systems focus on management strategies involving deadlock prevention, avoidance, detection, and recovery.
Prevention
Deadlock prevention aims to preclude one or more of the necessary conditions to make deadlocks impossible…
