Semaphores

Semaphores

This is the third part of the chapter which describes synchronization primitives in the Linux kernel and in the previous part we saw special type of spinlocks - queued spinlocks. The previous part was the last part which describes spinlocks related stuff. So we need to go ahead.

The next synchronization primitive after spinlock which we will see in this part is semaphore. We will start from theoretical side and will learn what is it semaphore and only after this, we will see how it is implemented in the Linux kernel as we did in the previous part.

So, let's start.

Introduction to the semaphores in the Linux kernel

So, what is it semaphore? As you may guess - semaphore is yet another mechanism for support of thread or process synchronization. The Linux kernel already provides implementation of one synchronization mechanism - spinlocks, why do we need in yet another one? To answer on this question we need to know details of both of these mechanisms. We already familiar with the spinlocks, so let's start from this mechanism.

spinlock creates a lock which will be acquired to protect a shared resource from being modified by more than one process. As a result, other processes that try to acquire the current lock get stopped (aka "spin-in-place" or busy waiting). Context switch is not allowed because preemption is disabled to avoid deadlocks. As a result, spinlock should only be used if the lock will only be acquired for a very short period of time, otherwise amount of busy waiting accumulated by other processes results in extremely inefficient operation. For locks that need to be acquired for a relatively long period of time, we turn to semaphore.

semaphores is a good solution for locks which may be acquired for a long time. In other way this mechanism is not optimal for locks that acquired for a short time. To understand this, we need to know what is semaphore.

As usual synchronization primitive, a semaphore is based on a variable. This variable may be incremented or decremented and it's state will represent ability to acquire lock. Notice that value of the variable is not limited to 0 and 1. There are two types of semaphores:

  • binary semaphore;

  • normal semaphore.

In the first case, value of semaphore may be only 1 or 0. In the second case value of semaphore any non-negative number. If the value of semaphore is greater than 1 it is called as counting semaphore and it allows to acquire a lock to more than 1 process. This allows us to keep records of available resources, when spinlock allows to hold a lock only on one task. Besides all of this, one more important thing that semaphore allows to sleep. Moreover when processes waits for a lock which is acquired by other process, the scheduler may switch on another process.

Semaphore API

So, we know a little about semaphores from theoretical side, let's look on its implementation in the Linux kernel. All semaphore API is located in the include/linux/semaphore.h header file.

We may see that the semaphore mechanism is represented by the following structure:

struct semaphore {
	raw_spinlock_t		lock;
	unsigned int		count;
	struct list_head	wait_list;
};

in the Linux kernel. The semaphore structure consists of three fields:

  • lock - spinlock for a semaphore data protection;

  • count - amount available resources;

  • wait_list - list of processes which are waiting to acquire a lock.

Before we will consider an API of the semaphore mechanism in the Linux kernel, we need to know how to initialize a semaphore. Actually the Linux kernel provides two approaches to execute initialization of the given semaphore structure. These methods allows to initialize a semaphore in a:

  • statically;

  • dynamically.

ways. Let's look at the first approach. We are able to initialize a semaphore statically with the DEFINE_SEMAPHORE macro:

#define DEFINE_SEMAPHORE(name)  \
         struct semaphore name = __SEMAPHORE_INITIALIZER(name, 1)

as we may see, the DEFINE_SEMAPHORE macro provides ability to initialize only binary semaphore. The DEFINE_SEMAPHORE macro expands to the definition of the semaphore structure which is initialized with the __SEMAPHORE_INITIALIZER macro. Let's look at the implementation of this macro:

#define __SEMAPHORE_INITIALIZER(name, n)              \
{                                                                       \
        .lock           = __RAW_SPIN_LOCK_UNLOCKED((name).lock),        \
        .count          = n,                                            \
        .wait_list      = LIST_HEAD_INIT((name).wait_list),             \
}

The __SEMAPHORE_INITIALIZER macro takes the name of the future semaphore structure and does initialization of the fields of this structure. First of all we initialize a spinlock of the given semaphore with the __RAW_SPIN_LOCK_UNLOCKED macro. As you may remember from the previous parts, the __RAW_SPIN_LOCK_UNLOCKED is defined in the include/linux/spinlock_types.h header file and expands to the __ARCH_SPIN_LOCK_UNLOCKED macro which just expands to zero or unlocked state:

#define __ARCH_SPIN_LOCK_UNLOCKED       { { 0 } }

The last two fields of the semaphore structure count and wait_list are initialized with the given value which represents count of available resources and empty list.

The second way to initialize a semaphore structure is to pass the semaphore and number of available resources to the sema_init function which is defined in the include/linux/semaphore.h header file:

static inline void sema_init(struct semaphore *sem, int val)
{
       static struct lock_class_key __key;
       *sem = (struct semaphore) __SEMAPHORE_INITIALIZER(*sem, val);
       lockdep_init_map(&sem->lock.dep_map, "semaphore->lock", &__key, 0);
}

Let's consider implementation of this function. It looks pretty easy and actually it does almost the same. Thus function executes initialization of the given semaphore with the __SEMAPHORE_INITIALIZER macro which we just saw. As I already wrote in the previous parts of this chapter, we will skip the stuff which is related to the lock validator of the Linux kernel.

So, from now we are able to initialize a semaphore let's look at how to lock and unlock. The Linux kernel provides following API to manipulate semaphores:

void down(struct semaphore *sem);
void up(struct semaphore *sem);
int  down_interruptible(struct semaphore *sem);
int  down_killable(struct semaphore *sem);
int  down_trylock(struct semaphore *sem);
int  down_timeout(struct semaphore *sem, long jiffies);

The first two functions: down and up are for acquiring and releasing of the given semaphore. The down_interruptible function tries to acquire a semaphore. If this try was successful, the count field of the given semaphore will be decremented and lock will be acquired, in other way the task will be switched to the blocked state or in other words the TASK_INTERRUPTIBLE flag will be set. This TASK_INTERRUPTIBLE flag means that the process may returned to ruined state by signal.

The down_killable function does the same as the down_interruptible function, but set the TASK_KILLABLE flag for the current process. This means that the waiting process may be interrupted by the kill signal.

The down_trylock function is similar on the spin_trylock function. This function tries to acquire a lock and exit if this operation was unsuccessful. In this case the process which wants to acquire a lock, will not wait. The last down_timeout function tries to acquire a lock. It will be interrupted in a waiting state when the given timeout will be expired. Additionally, you may notice that the timeout is in jiffies

We just saw definitions of the semaphore API. We will start from the down function. This function is defined in the kernel/locking/semaphore.c source code file. Let's look on the implementation function:

void down(struct semaphore *sem)
{
        unsigned long flags;

        raw_spin_lock_irqsave(&sem->lock, flags);
        if (likely(sem->count > 0))
                sem->count--;
        else
                __down(sem);
        raw_spin_unlock_irqrestore(&sem->lock, flags);
}
EXPORT_SYMBOL(down);

We may see the definition of the flags variable at the beginning of the down function. This variable will be passed to the raw_spin_lock_irqsave and raw_spin_lock_irqrestore macros which are defined in the include/linux/spinlock.h header file and protect a counter of the given semaphore here. Actually both of these macro do the same that spin_lock and spin_unlock macros, but additionally they save/restore current value of interrupt flags and disables interrupts.

As you already may guess, the main work is done between the raw_spin_lock_irqsave and raw_spin_unlock_irqrestore macros in the down function. We compare the value of the semaphore counter with zero and if it is bigger than zero, we may decrement this counter. This means that we already acquired the lock. In other way counter is zero. This means that all available resources already finished and we need to wait to acquire this lock. As we may see, the __down function will be called in this case.

The __down function is defined in the same source code file and its implementation looks:

static noinline void __sched __down(struct semaphore *sem)
{
        __down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
}

The __down function just calls the __down_common function with three parameters:

  • semaphore;

  • flag - for the task;

  • timeout - maximum timeout to wait semaphore.

Before we will consider implementation of the __down_common function, notice that implementation of the down_trylock, down_timeout and down_killable functions based on the __down_common too:

static noinline int __sched __down_interruptible(struct semaphore *sem)
{
        return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
}

The __down_killable:

static noinline int __sched __down_killable(struct semaphore *sem)
{
        return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
}

And the __down_timeout:

static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
{
        return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
}

Now let's look at the implementation of the __down_common function. This function is defined in the kernel/locking/semaphore.c source code file too and starts from the definition of the two following local variables:

struct task_struct *task = current;
struct semaphore_waiter waiter;

The first represents current task for the local processor which wants to acquire a lock. The current is a macro which is defined in the arch/x86/include/asm/current.h header file:

#define current get_current()

Where the get_current function returns value of the current_task per-cpu variable:

DECLARE_PER_CPU(struct task_struct *, current_task);

static __always_inline struct task_struct *get_current(void)
{
        return this_cpu_read_stable(current_task);
}

The second variable is waiter represents an entry of a semaphore.wait_list list:

struct semaphore_waiter {
        struct list_head list;
        struct task_struct *task;
        bool up;
};

Next we add current task to the wait_list and fill waiter fields after definition of these variables:

list_add_tail(&waiter.list, &sem->wait_list);
waiter.task = task;
waiter.up = false;

In the next step we join into the following infinite loop:

for (;;) {
        if (signal_pending_state(state, task))
            goto interrupted;

        if (unlikely(timeout <= 0))
            goto timed_out;

        __set_task_state(task, state);

        raw_spin_unlock_irq(&sem->lock);
        timeout = schedule_timeout(timeout);
        raw_spin_lock_irq(&sem->lock);

        if (waiter.up)
            return 0;
}

In the previous piece of code we set waiter.up to false. So, a task will spin in this loop while up will not be set to true. This loop starts from the check that the current task is in the pending state or in other words flags of this task contains TASK_INTERRUPTIBLE or TASK_WAKEKILL flag. As I already wrote above a task may be interrupted by signal during wait of ability to acquire a lock. The signal_pending_state function is defined in the include/linux/sched.h source code file and looks:

static inline int signal_pending_state(long state, struct task_struct *p)
{
         if (!(state & (TASK_INTERRUPTIBLE | TASK_WAKEKILL)))
                 return 0;
         if (!signal_pending(p))
                 return 0;

         return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
}

We check that the state bitmask contains TASK_INTERRUPTIBLE or TASK_WAKEKILL bits and if the bitmask does not contain this bit we exit. At the next step we check that the given task has a pending signal and exit if there is not. In the end we just check TASK_INTERRUPTIBLE bit in the state bitmask again or the SIGKILL signal. So, if our task has a pending signal, we will jump at the interrupted label:

interrupted:
    list_del(&waiter.list);
    return -EINTR;

where we delete task from the list of lock waiters and return the -EINTR error code. If a task has no pending signal, we check the given timeout and if it is less or equal zero:

if (unlikely(timeout <= 0))
    goto timed_out;

we jump at the timed_out label:

timed_out:
    list_del(&waiter.list);
    return -ETIME;

Where we do almost the same that we did in the interrupted label. We delete task from the list of lock waiters, but return the -ETIME error code. If a task has no pending signal and the given timeout is not expired yet, the given state will be set in the given task:

__set_task_state(task, state);

and call the schedule_timeout function:

raw_spin_unlock_irq(&sem->lock);
timeout = schedule_timeout(timeout);
raw_spin_lock_irq(&sem->lock);

which is defined in the kernel/time/timer.c source code file. The schedule_timeout function makes the current task sleep until the given timeout.

That is all about the __down_common function. A task which wants to acquire a lock which is already acquired by another task will be spun in the infinite loop while it will not be interrupted by a signal, the given timeout will not be expired or the task which holds a lock will not release it. Now let's look at the implementation of the up function.

The up function is defined in the same source code file as down function. As we already know, the main purpose of this function is to release a lock. This function looks:

void up(struct semaphore *sem)
{
        unsigned long flags;

        raw_spin_lock_irqsave(&sem->lock, flags);
        if (likely(list_empty(&sem->wait_list)))
                sem->count++;
        else
                __up(sem);
        raw_spin_unlock_irqrestore(&sem->lock, flags);
}
EXPORT_SYMBOL(up);

It looks almost the same as the down function. There are only two differences here. First of all we increment a counter of a semaphore if the list of waiters is empty. In other way we call the __up function from the same source code file. If the list of waiters is not empty we need to allow the first task from the list to acquire a lock:

static noinline void __sched __up(struct semaphore *sem)
{
        struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
                                                struct semaphore_waiter, list);
        list_del(&waiter->list);
        waiter->up = true;
        wake_up_process(waiter->task);
}

Here we takes the first task from the list of waiters, delete it from the list, set its waiter-up to true. From this point the infinite loop from the __down_common function will be stopped. The wake_up_process function will be called in the end of the __up function. As you remember we called the schedule_timeout function in the infinite loop from the __down_common this function. The schedule_timeout function makes the current task sleep until the given timeout will not be expired. So, as our process may sleep right now, we need to wake it up. That's why we call the wake_up_process function from the kernel/sched/core.c source code file.

That's all.

Conclusion

This is the end of the third part of the synchronization primitives chapter in the Linux kernel. In the two previous parts we already met the first synchronization primitive spinlock provided by the Linux kernel which is implemented as ticket spinlock and used for a very short time locks. In this part we saw yet another synchronization primitive - semaphore which is used for long time locks as it leads to context switch. In the next part we will continue to dive into synchronization primitives in the Linux kernel and will see next synchronization primitive - mutex.

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