Unreliable Guide To Locking

Table of Contents

1. Introduction

2. The Problem With Concurrency

Race Conditions and Critical Regions

3. Locking in the Linux Kernel

Two Main Types of Kernel Locks: Spinlocks and Mutexes

Locks and Uniprocessor Kernels

Locking Only In User Context

Locking Between User Context and Softirqs

Locking Between User Context and Tasklets

Locking Between User Context and Timers

Locking Between Tasklets/Timers

The Same Tasklet/Timer

Different Tasklets/Timers

Locking Between Softirqs

The Same Softirq

Different Softirqs

4. Hard IRQ Context

Locking Between Hard IRQ and Softirqs/Tasklets

Locking Between Two Hard IRQ Handlers

5. Cheat Sheet For Locking

Table of Minimum Requirements

6. The trylock Functions

7. Common Examples

All In User Context

Accessing From Interrupt Context

Exposing Objects Outside This File

Using Atomic Operations For The Reference Count

Protecting The Objects Themselves

8. Common Problems

Deadlock: Simple and Advanced

Preventing Deadlock

Overzealous Prevention Of Deadlocks

Racing Timers: A Kernel Pastime

9. Locking Speed

Read/Write Lock Variants

Avoiding Locks: Read Copy Update

Per-CPU Data

Data Which Mostly Used By An IRQ Handler

10. What Functions Are Safe To Call From Interrupts?

Some Functions Which Sleep

Some Functions Which Don't Sleep

11. Mutex API reference

mutex_init — initialize the mutex

mutex_is_locked — is the mutex locked

mutex_trylock_recursive — trylock variant that allows recursive locking

mutex_lock — acquire the mutex

mutex_unlock — release the mutex

ww_mutex_unlock — release the w/w mutex

mutex_lock_interruptible — acquire the mutex, interruptible

mutex_trylock — try to acquire the mutex, without waiting

atomic_dec_and_mutex_lock — return holding mutex if we dec to 0

12. Futex API reference

struct futex_q — The hashed futex queue entry, one per waiting task

hash_futex — Return the hash bucket in the global hash

match_futex — Check whether two futex keys are equal

get_futex_key — Get parameters which are the keys for a futex

fault_in_user_writeable — Fault in user address and verify RW access

futex_top_waiter — Return the highest priority waiter on a futex

futex_lock_pi_atomic — Atomic work required to acquire a pi aware futex

__unqueue_futex — Remove the futex_q from its futex_hash_bucket

requeue_futex — Requeue a futex_q from one hb to another

requeue_pi_wake_futex — Wake a task that acquired the lock during requeue

futex_proxy_trylock_atomic — Attempt an atomic lock for the top waiter

futex_requeue — Requeue waiters from uaddr1 to uaddr2

queue_me — Enqueue the futex_q on the futex_hash_bucket

unqueue_me — Remove the futex_q from its futex_hash_bucket

fixup_owner — Post lock pi_state and corner case management

futex_wait_queue_me queue_me and wait for wakeup, timeout, or signal

futex_wait_setup — Prepare to wait on a futex

handle_early_requeue_pi_wakeup — Detect early wakeup on the initial futex

futex_wait_requeue_pi — Wait on uaddr and take uaddr2

sys_set_robust_list — Set the robust-futex list head of a task

sys_get_robust_list — Get the robust-futex list head of a task

13. Further reading

14. Thanks

Glossary