Mutex (and RT Mutex) (Slideshow)

Locking in the Kernel

Userspace parallelism is simple …

  • All code is preemptible

  • … no way to disable preemption

  • Critical sections are best protected by a mutex pthread_mutex_t)

Kernel parallelism is different …

  • Schedulable code

    • Processes in kernel mode

    • kernel threads

  • Non-schedulable code

    • Interrupt service routines

    • Other atomic code (spinlock holders)

Mutual Exclusion: Mutex

Process context vs. process context

  • Classic mutex semantics

  • Binary semaphore

  • If held, arriving processes have to wait - they are scheduled*

#include <linux/mutex.h>
struct mutex mutex;
OO-like constructor and destructor
mutex_init(&mutex);
mutex_destroy(&mutex);

Mutex: Locking (1)

Locking is done in many different ways …

mutex_lock(&mutex);

Preferred version: interruptible

int error = mutex_lock_interruptible(&mutex);

  • Puts the caller to sleep if lock is held by someone else

    • Attention: no protection against self-deadlock!

  • “Interruptible”: return -EINTR (“Interrupted system call”) if process receives a signal

    • Good old Unix

  • Uninterruptible sleeps should be used with care

Mutex: Locking (2)

Recursive locking …

int error = mutex_lock_interruptible_nested(&mutex);

  • Same process may lock multiple times (no deadlock)

  • Must unlock as many times

  • Use is questionable though

Polling …

int error = mutex_trylock(&mutex);

  • Lock if not held

  • Otherwise, return -EAGAIN immediately

  • Use is even more questionable than recursive

Mutex: Releasing

At the end of the critical section …

mutex_unlock(&mutex);

  • Releases the lock

  • Wakes up waiter if any

Realtime Mutex

``struct mutex`` does not support priority inheritance

Linus Torvalds does not like realtime …

“Friends don’t let friends use priority inheritance. Just don’t do it. If you really need it, your system is broken anyway.”

  • LWN article

  • Features from the PREEMPT_RT tree keep trickling in

  • ⟶ “Realtime” mutex with priority inheritance

  • Used just like ordinary mutex

#include <linux/rtmutex.h>
struct rt_mutex mutex;