Usually a user process can hang inside kernel if it makes a system call and is blocked inside kernel waiting on a semaphore (where it would leave CPU) or spinning for a spinlock which might lock down the CPU core. You can examine where process is stuck by:

# echo w > /proc/sysrq-trigger

This sysrq will show you hung tasks and their stack trace in /var/log/messages or dmesg output.

But a process might as well hang in user mode e.g. on user mode semaphore. Most of the time there are some logical bugs in the program or kernel subsystems for which hang occurs which needs to be fixed instead of just restarting the process. Might work for user mode hang, but may not for hang inside kernel.

Rajat

On Thu, Dec 5, 2013 at 8:34 PM, Vipul Jain <vipulsj@gmail.com> wrote:

Hi All,

A quick question:
If any of user space process is hanged or not responding, does it means kernel is also hung in some part? I trying to understand why hang happens and what triggers it.happening ? How to go about recover a hang process, apart from restarting it.

Regards,
Vipul.

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