Mutex locks in Operating Systems - Step-by-Step Execution

Concept Flow - Mutex locks

Thread wants to enter critical section

↓

Check if mutex is unlocked

↓

Lock mutex

↓

Enter critical section

↓

Perform task

↓

Unlock mutex

↓

Other waiting threads check mutex again

This flow shows how a thread tries to lock a mutex before entering a critical section, waits if locked, then unlocks it after finishing.

Execution Sample

Operating Systems

mutex.lock()
// critical section code
mutex.unlock()

A thread locks the mutex, runs critical code, then unlocks the mutex to allow others.

Analysis Table

Step	Thread Action	Mutex State Before	Mutex State After	Thread State
1	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
2	Thread executes critical section	Locked	Locked	Working inside critical section
3	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
4	Another thread tries to lock mutex	Unlocked	Locked	Enters critical section
5	Another thread unlocks mutex	Locked	Unlocked	Leaves critical section
6	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
7	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
8	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
9	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
10	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
11	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
12	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
13	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
14	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
15	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
16	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
17	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
18	Thread tries to lock mutex	Unlocked	Locked	Enters critical section
19	Thread unlocks mutex	Locked	Unlocked	Leaves critical section
20	No more threads waiting	Unlocked	Unlocked	All done

💡 Execution stops because no threads are waiting and mutex is unlocked.

State Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 5	After Step 20
Mutex State	Unlocked	Locked	Unlocked	Locked	Unlocked	Unlocked
Thread State	Idle	In critical section	Idle	In critical section	Idle	Idle

Key Insights - 3 Insights

Why does a thread wait if the mutex is already locked?

Because the mutex ensures only one thread accesses the critical section at a time. The thread waits until the mutex becomes unlocked, as shown in execution_table rows 4 and 5.

What happens if a thread forgets to unlock the mutex?

Other threads will wait forever because the mutex stays locked. This causes a deadlock, preventing progress. The execution_table shows mutex unlocking at steps 3, 5, etc., which is necessary.

Can multiple threads be in the critical section simultaneously?

No, the mutex lock prevents this. Only one thread can hold the lock and enter the critical section at a time, as seen in the mutex state changes in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at Step 1. What is the mutex state after the thread tries to lock it?

A Unlocked

B Locked

C Waiting

D Undefined

💡 Hint

Check the 'Mutex State After' column at Step 1 in the execution_table.

At which step does the mutex become unlocked after the first thread finishes?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Mutex State After' column for steps where the thread unlocks the mutex.

If a thread never calls unlock(), what would happen to the mutex state in variable_tracker?

A It would remain unlocked

B It would toggle between locked and unlocked

C It would remain locked

D It would become undefined

💡 Hint

Refer to the key moment about forgetting to unlock and the mutex state changes in variable_tracker.

Concept Snapshot

Mutex locks control access to shared resources.
Only one thread can lock the mutex at a time.
Threads wait if mutex is locked.
Unlocking mutex allows others to proceed.
Prevents race conditions and data corruption.

Full Transcript

Mutex locks are used to control access to shared resources in operating systems. When a thread wants to enter a critical section, it tries to lock the mutex. If the mutex is unlocked, the thread locks it and enters the critical section. If the mutex is already locked by another thread, the thread waits until the mutex becomes unlocked. After finishing its task, the thread unlocks the mutex, allowing other waiting threads to lock it and enter the critical section. This process prevents multiple threads from accessing the critical section simultaneously, avoiding data corruption and race conditions. The execution table shows step-by-step how the mutex state changes and how threads enter and leave the critical section. Key moments highlight common confusions such as why threads wait, the importance of unlocking, and the exclusivity of the critical section. The visual quiz tests understanding of mutex state changes and consequences of incorrect usage.