Spin_lock 和互斥锁顺序

Question

I got test question (interview).

You need to grab both spin_lock and mutex in order to do something. What is the correct order of acquiring? Why?

I have some thoughts about this but no strong opinion about answer.

Answer 1

The reasone why you should grub lock is protect "critical region" on SMP or "critical region" on single CPU from preemption relative corruption (race). It is very important you machine type SMP or single CPU. It is also important what code inside spin and mutex. Is there kmalloc, vmalloc, mem_cache_alloc, alloc_bootmem or function with __user memory access oe even usleep.

spin_lock - it is the simplest lock in /include/asm/spinlock.h. Only one thread can be locked inside spin_lock in the same time. Any other thread that will try get spin_lock will be spin on the same place (instruction) until previous thread will free spin_lock. Spined thread will not go to sleep. So in the same time with spin_lock you can have two or more threads that will do something (one work and one spin). It is impossible on single CPU machine. But very good work on SMP. Code section inside spin_lock should be small and fast. If you code should work on differnt machines try check CONFIG_SMP and CONFIG_PREEMPT.

mutex - on other hand work lick semaphore /inside/asm/semaphore.h but counter is one. If counter is one so only one thread can go inside mutex lock region. Any other thread that will try get lock will see counter is zero becouse one thrad inside. And thread will go to wait queue. It will be woken up when the mutex be released and counter equal one. Thread inside mutex lock can sleep. It can call memory allocation function and get userspace memory.

(SMP)So imagine that you got spinlock and next mutex lock. So only one thread can get first spin and next mutex. But potentially inside mutex lock code cant sleep and it is bad. Because mutex inside spin.

(SMP)If you will get mutex lock and next spin lock. The same situation only one thread can go inside lock region. But between mutex get lock and spinlock code can sleep and also between spin_unlock and mutex free lock it can sleep too. Spin lock will get less unsleep region and it is good.

Answer 2

TL;DR: Lock mutex first and then spin lock.

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First you need to avoid such situations and be very careful to avoid deadlocks .

Then, you should consider effects of locking. Mutex may cause thread to block and sleep, while spin lock may cause thread to occupy processor in busy waiting loop. So it is general recommendation to keep critical sections that own a spin lock short in time which leads to following rule of thumb: do not sleep (ie by locking mutex) while owning a spin lock or you will waste CPU time.