What is an uninterruptible process?
Sometimes whenever I write a program in Linux and it crashes due to a bug of some sort, it will become an uninterruptible process and continue running forever until I restart my computer (even if I log out). My questions are:
- What causes a process to become uninterruptible?
- How do I stop that from happening?
- This is probably a dumb question, but is there any way to interrupt it without restarting my computer?
Answer
An uninterruptible process is a process which happens to be in a system call (kernel function) that cannot be interrupted by a signal.
To understand what that means, you need to understand the concept of an interruptible system call. The classic example is read(). This is a system call that can take a long time (seconds) since it can potentially involve spinning up a hard drive, or moving heads. During most of this time, the process will be sleeping, blocking on the hardware.
While the process is sleeping in the system call, it can receive a Unix asynchronous signal (say, SIGTERM), then the following happens:
- The system call exits prematurely, and is set up to return -EINTR to user space.
- The signal handler is executed.
- If the process is still running, it gets the return value from the system call, and it can make the same call again.
Returning early from the system call enables the user space code to immediately alter its behavior in response to the signal. For example, terminating cleanly in reaction to SIGINT or SIGTERM.
On the other hand, some system calls are not allowed to be interrupted in this way. If the system calls stalls for some reason, the process can indefinitely remains in this unkillable state.
LWN ran a nice article that touched this topic in July.
To answer the original question:
How to prevent this from happening: figure out which driver is causing you trouble, and either stop using, or become a kernel hacker and fix it.
How to kill an uninterruptible process without rebooting: somehow make the system call terminate. Frequently the most effective manner to do this without hitting the power switch is to pull the power cord. You can also become a kernel hacker and make the driver use TASK_KILLABLE, as explained in the LWN article.