Condition Synchronisation

operating-systems concurrency

Definition

Condition Synchronisation

Condition synchronisation is a concurrency control mechanism that ensures a specific execution order between processes or threads. A process delays execution until a logical condition or event (triggered by another process) is satisfied.

Unlike mutual exclusion, which prevents simultaneous access to maintain data consistency, condition synchronisation concerns coordination of action sequences to enforce required ordering.

Orthogonality to Mutual Exclusion

Goal	Mutual Exclusion	Condition Synchronisation
Primary Purpose	Data consistency	Execution ordering
Logic	”Only one at a time"	"Wait until event X”
Constraint	No overlapping access	Defined action sequence

Combined Use

These mechanisms are orthogonal. A solution may require mutual exclusion to protect shared data and condition synchronisation to enforce correct ordering of operations on that data.

Mechanism

To enforce that action $C_1$ in $P_1$ completes before $C_2$ in $P_2$ begins:

Semaphore S = 0

Void P1()
    C1()
    S.signal()           // signal completion

Void P2()
    S.wait()             // wait for C1
    C2()

Implementation

Common Primitives

• Counting Semaphores: Values represent available units or signals
• Condition Variables: Used within monitors to wait for state changes
• Message Passing: Blocking receive until a message (the condition) arrives