My trivial userland spinlock implementation

spinlocks.h

#ifndef SPINLOCKS_H
#define SPINLOCKS_H

#include <unistd.h>

typedef volatile char spinlock_t;

#if defined(__x86_64__) || defined(__i386) || defined(_X86_)
# define CPU_PAUSE __asm__ ("pause")
#else
# define CPU_PAUSE (void) 0
#endif

#define SPINLOCK_LOCK(L) \
    do { \
        if (__sync_lock_test_and_set((L), 1) != 0) { \
            unsigned int spin_count = 0U; \
            for (;;) { \
                CPU_PAUSE; \
                if (*(L) == 0U && \
                    __sync_lock_test_and_set((L), 1) == 0) { \
                    break; \
                } \
                if (++spin_count > 100U) { \
                    usleep(0); \
                } \
            } \
        } \
        ANNOTATE_WRITERLOCK_ACQUIRED((const void *) (L)); \
    } while(0)

#define SPINLOCK_UNLOCK(L) \
    do { \
        __sync_lock_release(L); \
        ANNOTATE_WRITERLOCK_RELEASED((const void *) (L)); \
    } while(0)

#ifdef VALGRIND_INSTRUMENTATION
# include <valgrind/drd.h>
#endif
#ifndef ANNOTATE_WRITERLOCK_ACQUIRED
# define ANNOTATE_WRITERLOCK_ACQUIRED(A)   (void) 0
# define ANNOTATE_WRITERLOCK_RELEASED(A)   (void) 0
# define ANNOTATE_RWLOCK_CREATE(A)         (void) 0
#endif

#endif