# 2022 quiz6

contributed by <shawn5141>

EXP1 = parent_tid
EXP2 = child_tid

### Clone system call
#### CLONE_FLAGS is passed to clone system call.

#define CLONE_FLAGS                                                     \
    (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_THREAD | \
     CLONE_SYSVSEM | CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID)

這些 flag 指名 clone 需要擁有下列特性。
> CLONE_VM: the calling process and the child process run in the same memory space.
> CLONE_FS: the caller and the child process share the same filesystem information
> CLONE_FILES: the calling process and the child process share the same file descriptor table
> CLONE_SIGHAND: the calling process and the child process share the same table of signal handlers
> CLONE_THREAD: the child is placed in the same thread group as the calling process
> CLONE_SYSVSEM: then the child and the calling process share a single list of System V semaphore adjustment (semadj) values.
> CLONE_PARENT_SETTID: Store the child thread ID at the location pointed to by parent_tid.
> CLONE_CHILD_CLEARTID: Clear (zero) the child thread ID at the location pointed to by child_tid.

thread_t tid = clone(wrap, (char *) thread_stack + STACK_SZ + GUARD_SZ, CLONE_FLAGS, fa, &(node->tid), NULL, &(node->tid));

### spin_lock

### mutex

### futex

[futex 綜敘](http://blog.foool.net/2021/04/futex-%E7%BB%BC%E8%BF%B0/)
竞争态总是很少发生的，只有在竞争态才需要进入内核，否则在用户态即可完成。futex的两个目标是：1）尽量避免系统调用；2）避免不必要的上下文切换（导致的TLB失效等）。
> 注意：futex 在Linux 的内核实现为一个系统调用（SYS_futex），用户程序如果直接调用它肯定会进入内核态，它还需要和其他语句（如原子操作）配合使用，新手在未理解其futex 原理和并发控制机制时极易犯错，这也是为什么不推荐直接使用它的原因。

syscall(SYS_futex, m, FUTEX_WAIT, 1, NULL, NULL, 0);

[lock in golang video](https://www.youtube.com/watch?v=7OpCf6f_BAM&t=1047s)

ref: https://www.youtube.com/watch?v=wPfPuliEI5M