[Solved] In a producer-consumer implementation using semaphores, a candidate writes code where the mutex is acquired after the 'empty' semaphore is decremented but... What is the subtle bug in this approach? | Operating Systems

Operating Systems - Producer-Consumer Problem Using Semaphores

In a producer-consumer implementation using semaphores, a candidate writes code where the mutex is acquired after the 'empty' semaphore is decremented but released before the 'full' semaphore is incremented. What is the subtle bug in this approach?

AThe producer may deadlock waiting for the mutex held by the consumer

BThe 'empty' semaphore count may become negative causing underflow

CThe buffer size may be incorrectly updated leading to overflow

DThe consumer might consume an item before the producer finishes inserting it, causing race conditions

Step-by-Step Solution

Solution:

Step 1: Analyze mutex scope
Mutex must protect the entire critical section including insertion and signaling 'full'.
Step 2: Consequence of early mutex release
Releasing mutex before signaling 'full' allows consumer to proceed and access incomplete data.
Step 3: Evaluate other options
Deadlock (A) unlikely here; buffer size update (C) is protected by mutex; semaphore underflow (D) prevented by semaphore semantics.
Final Answer:
Option D -> Option D
Quick Check:
Mutex must cover insertion and signaling to avoid race [OK]

Quick Trick: Mutex must protect insertion and signaling to prevent race [OK]

Common Mistakes:

MISTAKES

Releasing mutex too early
Assuming semaphore signaling is atomic with insertion
Ignoring critical section boundaries

Trap Explanation:

PITFALL

Candidates often separate signaling from critical section, causing subtle race conditions.

Interviewer Note:

CONTEXT

Reveals candidate's understanding of critical section boundaries and semaphore signaling.

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Solution Trace

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What is the subtle bug in this approach?

Step 1: Analyze mutex scope

Step 2: Consequence of early mutex release

Step 3: Evaluate other options

Final Answer:

Quick Check:

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