Function rwlock_rdlock_impl

src/bthread/rwlock.cpp:42–74

For reading.

// For reading.
static int rwlock_rdlock_impl(bthread_rwlock_t* __restrict rwlock,
                              const struct timespec* __restrict abstime) {
    int reader_count = ((butil::atomic<int>*)&rwlock->reader_count)
        ->fetch_add(1, butil::memory_order_acquire) + 1;
    // Fast path.
    if (reader_count >= 0) {
        CHECK_LT(reader_count, RWLockMaxReaders);
        return 0;
    }

    // Slow path.

    // Don't sample when contention profiler is off.
    if (NULL == bthread::g_cp) {
        return bthread_sem_timedwait(&rwlock->reader_sema, abstime);
    }
    // Ask Collector if this (contended) locking should be sampled.
    const size_t sampling_range = bvar::is_collectable(&bthread::g_cp_sl);
    if (!bvar::is_sampling_range_valid(sampling_range)) { // Don't sample.
        return bthread_sem_timedwait(&rwlock->reader_sema, abstime);
    }

    // Sample.
    const int64_t start_ns = butil::cpuwide_time_ns();
    int rc = bthread_sem_timedwait(&rwlock->reader_sema, abstime);
    const int64_t end_ns = butil::cpuwide_time_ns();
    const bthread_contention_site_t csite{end_ns - start_ns, sampling_range};
    // Submit `csite' for each reader immediately after
    // owning rdlock to avoid the contention of `csite'.
    bthread::submit_contention(csite, end_ns);

    return rc;
}

static inline int rwlock_rdlock(bthread_rwlock_t* rwlock) {
    return rwlock_rdlock_impl(rwlock, NULL);