perf_mmap__read_head
u64 perf_mmap__read_head(struct perf_mmap *map);
u64 head = perf_mmap__read_head(md);
map->prev = perf_mmap__read_head(map);
map->end = perf_mmap__read_head(map);
return perf_mmap__read_head(map) == map->prev && !pc->aux_size;
u64 head = perf_mmap__read_head(&md->core);