Function zstd_mempool_alloc

freebsd/contrib/openzfs/module/zstd/zfs_zstd.c:247–336

 */

static void *
zstd_mempool_alloc(struct zstd_pool *zstd_mempool, size_t size)
{
	struct zstd_pool *pool;
	struct zstd_kmem *mem = NULL;

	if (!zstd_mempool) {
		return (NULL);
	}

	/* Seek for preallocated memory slot and free obsolete slots */
	for (int i = 0; i < ZSTD_POOL_MAX; i++) {
		pool = &zstd_mempool[i];
		/*
		 * This lock is simply a marker for a pool object beeing in use.
		 * If it's already hold, it will be skipped.
		 *
		 * We need to create it before checking it to avoid race
		 * conditions caused by running in a threaded context.
		 *
		 * The lock is later released by zstd_mempool_free.
		 */
		if (mutex_tryenter(&pool->barrier)) {
			/*
			 * Check if objects fits the size, if so we take it and
			 * update the timestamp.
			 */
			if (pool->mem && size <= pool->size) {
				pool->timeout = gethrestime_sec() +
				    ZSTD_POOL_TIMEOUT;
				mem = pool->mem;
				return (mem);
			}
			mutex_exit(&pool->barrier);
		}
	}

	/*
	 * If no preallocated slot was found, try to fill in a new one.
	 *
	 * We run a similar algorithm twice here to avoid pool fragmentation.
	 * The first one may generate holes in the list if objects get released.
	 * We always make sure that these holes get filled instead of adding new
	 * allocations constantly at the end.
	 */
	for (int i = 0; i < ZSTD_POOL_MAX; i++) {
		pool = &zstd_mempool[i];
		if (mutex_tryenter(&pool->barrier)) {
			/* Object is free, try to allocate new one */
			if (!pool->mem) {
				mem = vmem_alloc(size, KM_SLEEP);
				if (mem) {
					ZSTDSTAT_ADD(zstd_stat_buffers, 1);
					ZSTDSTAT_ADD(zstd_stat_size, size);
					pool->mem = mem;
					pool->size = size;
					/* Keep track for later release */
					mem->pool = pool;