Os Deadlock Lock Ordering

DodaTech 1 min read

In this tutorial, you'll learn about How to Fix Lock Ordering Errors. We cover key concepts, practical examples, and best practices.

Fix lock ordering errors when inconsistent lock acquisition order across threads.

Quick Fix

Wrong

import threading
class Transfer:
    def __init__(self):
        self.lock=threading.Lock()
    def transfer(self,other,amt):
        self.lock.acquire()
        other.lock.acquire()  # inconsistent order!
        self.balance-=amt; other.balance+=amt
        other.lock.release(); self.lock.release()

transfer(a,b) acquires a then b. transfer(b,a) acquires b then a. Circular wait -> deadlock.

Right

import threading
class Transfer:
    def __init__(self,id): self.id=id; self.lock=threading.Lock()
    def transfer(self,other,amt):
        # Always lock lower ID first
        first, second = (self, other) if id(self) < id(other) else (other, self)
        first.lock.acquire(); second.lock.acquire()
        self.balance-=amt; other.balance+=amt
        second.lock.release(); first.lock.release()
# Or use resource hierarchy:
lock1=threading.Lock(); lock2=threading.Lock()
def worker():
    with lock1:  # both threads acquire in same order
        with lock2: pass

Lock ordering prevents deadlock. Both transfers acquire locks in consistent order.

Prevention

Always acquire locks in global consistent order. Use resource hierarchy numbering.

DodaTech Tools

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FAQ

What is lock ordering?

Assign global order to locks. Always acquire in increasing order. Prevents circular wait.

How?

By hash, by ID, by numeric level. If all threads follow same order, no cycle.

Example?

Bank accounts: acquire lock with lower account number first. Baking: acquire flour then eggs.

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