vsync / spinlock / ticketlock.h

Classic ticketlock with two 32-bit variables (owner and next).

Groups: Fair locks

Ticketlock is a simple, fair lock (guarantees FIFO order).

Example:

#include <vsync/spinlock/ticketlock.h>
#include <vsync/common/assert.h>
#include <pthread.h>
#include <stdio.h>

#define N            12
#define EXPECTED_VAL N

ticketlock_t g_lock = TICKETLOCK_INIT();
vuint32_t g_x       = 0;
vuint32_t g_y       = 0;

void *
run(void *args)
{
    ticketlock_acquire(&g_lock);
    g_x++;
    g_y++;
    ticketlock_release(&g_lock);

    (void)args;
    return NULL;
}

int
main(void)
{
    pthread_t threads[N];

    for (vsize_t i = 0; i < N; i++) {
        pthread_create(&threads[i], NULL, run, (void *)i);
    }

    for (vsize_t i = 0; i < N; i++) {
        pthread_join(threads[i], NULL);
    }

    ASSERT(g_x == EXPECTED_VAL);
    ASSERT(g_x == g_y);
    printf("Final value %u\n", g_x);
    return 0;
}

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Functions

Function	Description
ticketlock_init	Initializes the ticketlock.
ticketlock_acquire	Acquires the ticketlock.
ticketlock_tryacquire	Tries to acquire the ticketlock.
ticketlock_release	Releases the ticketlock.
ticketlock_has_waiters	Checks whether there are threads waiting to acquire the lock.

Function ticketlock_init

static void ticketlock_init(ticketlock_t *l)

Initializes the ticketlock.

Parameters:

• l: address of ticketlock_t object.

Note: alternatively use TICKETLOCK_INIT.

Function ticketlock_acquire

static void ticketlock_acquire(ticketlock_t *l)

Acquires the ticketlock.

Parameters:

• l: address of ticketlock_t object.

Function ticketlock_tryacquire

static vbool_t ticketlock_tryacquire(ticketlock_t *l)

Tries to acquire the ticketlock.

Parameters:

• l: address of ticketlock_t object.

Returns: true, if lock is acquired successfully.

Returns: false, if failed to acquire the lock.

Function ticketlock_release

static void ticketlock_release(ticketlock_t *l)

Releases the ticketlock.

Parameters:

• l: address of ticketlock_t object.

Function ticketlock_has_waiters

static vbool_t ticketlock_has_waiters(ticketlock_t *l)

Checks whether there are threads waiting to acquire the lock.

This can only be called by the thread holding the lock, ie, after acquire but before release.

Returns: true, if threads are waiting to acquire the lock.

Returns: false, otherwise.

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