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What is a deadlock and how can it happen with threads? Deadlock क्या है और threads के साथ यह कैसे हो सकता है?

Answer

A deadlock occurs when two or more threads are each waiting for a resource held by the other, so none can proceed - the program freezes indefinitely.

import threading
import time

lock_a = threading.Lock()
lock_b = threading.Lock()

def task1():
    with lock_a:
        print('Task1 acquired lock_a')
        time.sleep(0.1)  # gives task2 time to acquire lock_b
        print('Task1 waiting for lock_b')
        with lock_b:  # blocks here if task2 already holds lock_b
            print('Task1 acquired lock_b')

def task2():
    with lock_b:
        print('Task2 acquired lock_b')
        time.sleep(0.1)
        print('Task2 waiting for lock_a')
        with lock_a:  # blocks here - DEADLOCK: each waits on what the other holds
            print('Task2 acquired lock_a')

# t1 = threading.Thread(target=task1)
# t2 = threading.Thread(target=task2)
# t1.start(); t2.start()
# Program hangs forever - classic deadlock

# FIX 1: always acquire locks in the SAME consistent order across all threads
def task1_fixed():
    with lock_a:
        with lock_b:
            print('Task1 done')

def task2_fixed():
    with lock_a:  # same order as task1_fixed - lock_a first, then lock_b
        with lock_b:
            print('Task2 done')

# FIX 2: use a timeout when acquiring locks to avoid indefinite blocking
acquired = lock_a.acquire(timeout=2)
if acquired:
    try:
        pass  # do work
    finally:
        lock_a.release()
else:
    print('Could not acquire lock in time, avoiding deadlock')

Deadlock तब होता है जब दो या ज़्यादा threads एक-दूसरे के पास मौजूद resource का इंतज़ार करते हैं, कोई आगे नहीं बढ़ पाता - program हमेशा के लिए freeze हो जाता है।

import threading
import time

lock_a = threading.Lock()
lock_b = threading.Lock()

def task1():
    with lock_a:
        print('Task1 ने lock_a लिया')
        time.sleep(0.1)
        with lock_b:  # यहां block होता है अगर task2 lock_b रखता है
            print('Task1 ने lock_b लिया')

def task2():
    with lock_b:
        print('Task2 ने lock_b लिया')
        time.sleep(0.1)
        with lock_a:  # DEADLOCK
            print('Task2 ने lock_a लिया')

# Program हमेशा के लिए hang हो जाता है

# FIX 1: सभी threads में locks एक ही consistent order में acquire करें
def task1_fixed():
    with lock_a:
        with lock_b:
            print('Task1 पूरा')

def task2_fixed():
    with lock_a:  # same order
        with lock_b:
            print('Task2 पूरा')

# FIX 2: timeout use करें
acquired = lock_a.acquire(timeout=2)
if acquired:
    try:
        pass
    finally:
        lock_a.release()
else:
    print('Lock time पर नहीं मिला, deadlock से बचा')

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