跨线程的 ucontext

发布于 2024-10-07 14:40:44 字数 228 浏览 7 评论 0原文

上下文（由 ucontext.h 中的函数操作的对象）是否允许跨线程共享？也就是说，我可以使用第二个参数来 swapcontext 来作为另一个线程上的 makecontext 创建的上下文吗？测试程序似乎表明这可以在 Linux 上运行。我找不到这方面的文档，而 Windows 光纤似乎明确支持这种用例。一般来说，这样做安全吗？这是标准的 POSIX 行为吗？

原文

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

风月客 2024-10-14 14:40:44

实际上，Linux 有一个 NGPT - 线程库，它使用的不是当前的 1:1 线程模型（每个用户线程都是内核线程或 LWP），而是 M:N 线程模型（多个用户线程对应于另一个更小的线程）内核线程数）。

根据 ftp://ftp.uni-duisburg.de/ Linux/NGPT/ngpt-0.9.4.tar.gz/ngpt-0.9.4/pth_sched.c:170 pth_scheduler 可以在本机（内核）线程之间移动用户线程上下文：

        /*
         * See if the thread is unbound...
         * Break out and schedule if so...
         */
        if (current->boundnative == 0)
            break;
        /*
         * See if the thread is bound to a different native thread...
         * Break out and schedule if not...
         */
        if (current->boundnative == this_sched->lastrannative)
            break;

保存和恢复用户线程时，可以使用 ucontext ftp://ftp.uni-duisburg.de/Linux/NGPT/ngpt-0.9.4.tar.gz/ngpt-0.9.4/pth_mctx.c:64 似乎这是一种首选方法 (mcsc)：

/*
 * save the current machine context
 */
#if PTH_MCTX_MTH(mcsc)
#define pth_mctx_save(mctx) \
        ( (mctx)->error = errno, \
          getcontext(&(mctx)->uc) )
#elif
....
/*
 * restore the current machine context
 * (at the location of the old context)
 */
#if PTH_MCTX_MTH(mcsc)
#define pth_mctx_restore(mctx) \
        ( errno = (mctx)->error, \
          (void)setcontext(&(mctx)->uc) )
#elif PTH_MCTX_MTH(sjlj)
...

#if PTH_MCTX_MTH(mcsc)

/*
 * VARIANT 1: THE STANDARDIZED SVR4/SUSv2 APPROACH
 *
 * This is the preferred variant, because it uses the standardized
 * SVR4/SUSv2 makecontext(2) and friends which is a facility intended
 * for user-space context switching. The thread creation therefore is
 * straight-foreward.
 */

因此，即使 NGPT 已失效且未使用，它也会选择 *context() 来切换用户线程，甚至在内核线程之间切换。我认为，在 Linux 上使用 *context() 系列是足够安全的。

混合 ucontexts 和其他本机线程库时可能会出现一些问题。我会考虑 NPTL，它是自 glibc 2.4 以来的标准 Linux 本机线程库。主要问题是 THREAD_SELF - 指向当前线程的 struct pthread 的指针。 TLS（线程本地存储）也通过 THREAD_SELF 工作。 THREAD_SELF 通常存储在寄存器中（r2 on powerpc，% x86 上的 gs 等）。 get/setcontext 可能会保存和恢复该寄存器，从而破坏本机 pthread 库的内部结构（例如线程本地存储、线程标识等）。

glibc setcontext 不会保存/恢复%gs 注册以与 pthread 兼容：

    /* Restore the FS segment register.  We don't touch the GS register
       since it is used for threads.  */
    movl    oFS(%eax), %ecx
    movw    %cx, %fs

您应该检查 setcontext 是否在您感兴趣的架构上保存 THREAD_SELF 注册。此外，您的代码不能在操作系统和 libc。

Actually, there was an NGPT - threading library for linux, which uses not a current 1:1 threading model (each user thread is the kernel thread or LWP), but a M:N threading model (several user threads corresponds to another, smaller number of kernel threads).

According to ftp://ftp.uni-duisburg.de/Linux/NGPT/ngpt-0.9.4.tar.gz/ngpt-0.9.4/pth_sched.c:170 pth_scheduler it was possible of moving user thread contexts between native (kernel) threads:

        /*
         * See if the thread is unbound...
         * Break out and schedule if so...
         */
        if (current->boundnative == 0)
            break;
        /*
         * See if the thread is bound to a different native thread...
         * Break out and schedule if not...
         */
        if (current->boundnative == this_sched->lastrannative)
            break;

To save and restore user threads, the ucontext can be used ftp://ftp.uni-duisburg.de/Linux/NGPT/ngpt-0.9.4.tar.gz/ngpt-0.9.4/pth_mctx.c:64 and seems this was a preferred method (mcsc):

/*
 * save the current machine context
 */
#if PTH_MCTX_MTH(mcsc)
#define pth_mctx_save(mctx) \
        ( (mctx)->error = errno, \
          getcontext(&(mctx)->uc) )
#elif
....
/*
 * restore the current machine context
 * (at the location of the old context)
 */
#if PTH_MCTX_MTH(mcsc)
#define pth_mctx_restore(mctx) \
        ( errno = (mctx)->error, \
          (void)setcontext(&(mctx)->uc) )
#elif PTH_MCTX_MTH(sjlj)
...

#if PTH_MCTX_MTH(mcsc)

/*
 * VARIANT 1: THE STANDARDIZED SVR4/SUSv2 APPROACH
 *
 * This is the preferred variant, because it uses the standardized
 * SVR4/SUSv2 makecontext(2) and friends which is a facility intended
 * for user-space context switching. The thread creation therefore is
 * straight-foreward.
 */

So, even if NGPT is dead and unused, it selected *context() for switching user threads even between kernel threads. I assume, that using *context() family is safe enough on Linux.

There can be some problems when mixing ucontexts and other native threads library. I will consider a NPTL, which is standard linux native threading library since glibc 2.4. The main problem is THREAD_SELF - pointer to struct pthread of the current thread. TLS (Thread-local storage) also works via THREAD_SELF. The THREAD_SELF is usually stored on register (r2 on powerpc, %gs on x86, etc). get/setcontext might save and restore this register breaking internals of native pthread library (e.g. thread-local storage, thread identification, etc).

The glibc setcontext will not save/restore %gs register to be compatible with pthreads:

    /* Restore the FS segment register.  We don't touch the GS register
       since it is used for threads.  */
    movl    oFS(%eax), %ecx
    movw    %cx, %fs

You should check, does setcontext saves THREAD_SELF register on the architecture you are interested in. Also, your code can be not portable between OSes and libcs.

回复收藏 0 原文