读/写阻塞系统调用是否将进程置于 TASK_UNINTERRUPTIBLE 或 TASK_INTERRUPTIBLE 状态？答案

【问题标题】：Does read/write blocked system call put the process in TASK_UNINTERRUPTIBLE or TASK_INTERRUPTIBLE state?读/写阻塞系统调用是否将进程置于 TASK_UNINTERRUPTIBLE 或 TASK_INTERRUPTIBLE 状态？
【发布时间】：2019-10-01 19:33:46
【问题描述】：

如 signal(7) 的手册页中所述，

   Interruption of system calls and library functions by signal handlers
       If a signal handler is invoked while a system call or library function call is blocked, then either:

       * the call is automatically restarted after the signal handler returns; or

       * the call fails with the error EINTR.

       Which  of  these  two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag (see sigaction(2)).  The details vary across UNIX systems; below, the details for
       Linux.

       If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call will be automatically restarted after the signal handler returns if the SA_RESTART flag was used; otherwise the call  will
       fail with the error EINTR:

           * read(2),  readv(2),  write(2), writev(2), and ioctl(2) calls on "slow" devices.  A "slow" device is one where the I/O call may block for an indefinite time, for example, a terminal, pipe, or socket.  If an I/O call on a
             slow device has already transferred some data by the time it is interrupted by a signal handler, then the call will return a success status (normally, the number of bytes transferred).  Note that a (local) disk is not a
             slow device according to this definition; I/O operations on disk devices are not interrupted by signals.

如前所述，对以下接口之一的阻塞调用（读、写）被信号处理程序中断，如果使用了 SA_RESTART 标志，则在信号处理程序返回后，调用将自动重新启动，这意味着在阻塞读/写系统调用的情况下，进程必须处于 TASK_INTERRUPTIBLE 状态。

但是当我试图找出将进程置于 TASK_UNINTERRUPTIBLE 状态的阻塞系统调用时，我发现了 https://unix.stackexchange.com/questions/62697/why-is-i-o-uninterruptible 和 Why doing I/O in Linux is uninterruptible? ，并且在这两个地方都提到了阻塞的 I/O 调用（读、写）将在 TASK_UNINTERRUPTIBLE 中放置一个进程。

这里也提到了：https://access.redhat.com/sites/default/files/attachments/processstates_20120831.pdf

The Uninterruptible state is mostly used by device drivers waiting for disk or network I/O. When the process
is sleeping uninterruptibly, signals accumulated during the sleep are noticed when the process returns from
the system call or trap. In Linux systems. the command ps -l uses the letter D in the state field (S) to
indicate that the process is in an Uninterruptible sleep state. In that case, the process state flag is set as
follows:
p->state = TASK_UNINTERRUPTABLE
LEARN MORE: Read more about D states in the Red Hat Knowledgebase:
https://access.redhat.com/knowledge/solutions/59989/

这有点令人困惑。

我还想知道其他可以将进程置于 TASK_UNINTERRUPTIBLE 状态的阻塞系统调用。

【问题讨论】：

标签： linux linux-kernel kernel signals system-calls

【解决方案1】：

对于read(2) 或write(2) 系列系统调用，睡眠类型取决于正在访问的文件类型。在您引用的文档中，“慢速”设备是那些read/write 将中断睡眠的设备，而“快速”设备是那些将不间断睡眠的设备（不间断睡眠状态被命名为D 用于“磁盘等待”，因为最初磁盘文件上的read/write 是此类睡眠的最常见原因。

请注意，“阻塞”在技术上仅指可中断睡眠。

几乎任何系统调用都可以进入不间断睡眠，因为当进程需要获取保护内部内核资源的锁时（除其他外），这可能会发生。通常，这种不间断的睡眠非常短暂，您不会注意到它。

【讨论】：