TASKUNINTERRUPTIBLE

TASK_UNINTERRUPTIBLE, often referred to as the TASK_UNINTERRUPTIBLE state (commonly shown as the D state in user space), is a Linux kernel task state describing a process that is sleeping and cannot be interrupted by signals. This state is entered when a task must wait for a non‑interruptible condition, such as completion of an I/O operation or a driver/resource lock. While in this state, the task is not scheduled to run until the awaited condition becomes true and the kernel explicitly wakes it up.

A key characteristic of TASK_UNINTERRUPTIBLE is that signals do not wake the task. The only way for the process to resume is when the underlying event completes or when the kernel performs a wake_up() path. Because signals cannot interrupt it, a task in this state can cause system latency if the awaited condition hangs or fails to complete.

In user-space visibility, processes in an uninterruptible sleep are shown with a D in the process status, for example State: D (uninterruptible sleep) in /proc/[pid]/status. This contrasts with TASK_INTERRUPTIBLE, where a sleeping task can be awakened by signals and is marked with an S state.

Common causes include waiting on disk I/O, memory-mapped I/O, or certain hardware drivers that hold a lock or await hardware readiness. Prolonged D-state often points to underlying I/O or driver issues rather than to ordinary scheduling delays.

Related concepts include TASK_INTERRUPTIBLE and TASK_RUNNING. Diagnosing D-state conditions involves examining I/O wait times, disk and device health, and kernel driver behavior, using tools like ps, top, iostat, and kernel logs to identify the source of the uninterruptible wait.