What Is an Uninterruptible Process

What is an uninterruptible process?

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.

How to stop 'uninterruptible' process on Linux?

Simple answer: you cannot.

Longer answer: the uninterruptable sleep means the process will not be woken up by signals. It can be only woken up by what it's waiting for. When I get such situations eg. with CD-ROM, I usually reset the computer by using suspend-to-disk and resuming.

How can I put process into uninterruptible sleep?

To put a process in an interruptible sleep mode you will have to go into kernel code.
Using the functionality of wake_up_interruptible and spin locks you will be able to achieve this state.

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