What is an uninterruptible process?

When a process is on user mode, it can be interrupted at any time (switching to kernel mode). When the kernel returns to user mode, it checks if there are any signals pending (including the ones which are used to kill the process, such as SIGTERM and SIGKILL). This means a process can be killed only on return to user mode.

The reason a process cannot be killed in kernel mode is that it could potentially corrupt the kernel structures used by all the other processes in the same machine (the same way killing a thread can potentially corrupt data structures used by other threads in the same process).

When the kernel needs to do something which could take a long time (waiting on a pipe written by another process or waiting for the hardware to do something, for instance), it sleeps by marking itself as sleeping and calling the scheduler to switch to another process (if there is no non-sleeping process, it switches to a "dummy" process which tells the cpu to slow down a bit and sits in a loop — the idle loop).

If a signal is sent to a sleeping process, it has to be woken up before it will return to user space and thus process the pending signal. Here we have the difference between the two main types of sleep:

• TASK_INTERRUPTIBLE, the interruptible sleep. If a task is marked with this flag, it is sleeping, but can be woken by signals. This means the code which marked the task as sleeping is expecting a possible signal, and after it wakes up will check for it and return from the system call. After the signal is handled, the system call can potentially be automatically restarted (and I won't go into details on how that works).
• TASK_UNINTERRUPTIBLE, the uninterruptible sleep. If a task is marked with this flag, it is not expecting to be woken up by anything other than whatever it is waiting for, either because it cannot easily be restarted, or because programs are expecting the system call to be atomic. This can also be used for sleeps known to be very short.

TASK_KILLABLE (mentioned in the LWN article linked to by ddaa's answer) is a new variant.

This answers your first question. As to your second question: you can't avoid uninterruptible sleeps, they are a normal thing (it happens, for instance, every time a process reads/writes from/to the disk); however, they should last only a fraction of a second. If they last much longer, it usually means a hardware problem (or a device driver problem, which looks the same to the kernel), where the device driver is waiting for the hardware to do something which will never happen. It can also mean you are using NFS and the NFS server is down (it is waiting for the server to recover; you can also use the "intr" option to avoid the problem).

Finally, the reason you cannot recover is the same reason the kernel waits until return to user mode to deliver a signal or kill the process: it would potentially corrupt the kernel's data structures (code waiting on an interruptible sleep can receive an error which tells it to return to user space, where the process can be killed; code waiting on an uninterruptible sleep is not expecting any error).

Uninterruptable processes are USUALLY waiting for I/O following a page fault.

Consider this:

• The thread tries to access a page which is not in core (either an executable which is demand-loaded, a page of anonymous memory which has been swapped out, or a mmap()'d file which is demand loaded, which are much the same thing)
• The kernel is now (trying to) load it in
• The process can't continue until the page is available.

The process/task cannot be interrupted in this state, because it can't handle any signals; if it did, another page fault would happen and it would be back where it was.

When I say "process", I really mean "task", which under Linux (2.6) roughly translates to "thread" which may or may not have an individual "thread group" entry in /proc

In some cases, it may be waiting for a long time. A typical example of this would be where the executable or mmap'd file is on a network filesystem where the server has failed. If the I/O eventually succeeds, the task will continue. If it eventually fails, the task will generally get a SIGBUS or something.

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.

To your 3rd question: I think you can kill the uninterruptable processes by running sudo kill -HUP 1. It will restart init without ending the running processes and after running it, my uninterruptable processes were gone.