Kernel Lock Debugging

[Daniel Baluta]

Hello,
> i wrote a linux kernel module which uses mmap_sem in lots of places. I grab
> mmap_sem for both reads & writes.  And, i also use page_table_lock spinlock
> under them. The problem i have is that there is a hard to reproduce a race
> condition bug in which a thread doesn't release the semaphore at all. Hence,
> ps hangs as ps needs to grab mmap_sem for the processes. The question is how
> do i find the thread that is holding the semaphore forever. It will help me
> figure out the part of the code the race is in.
> So, the question is : I have some number of threads grabbing the semaphore
> for read/write. One of them grabs it & doesn't release at all. How do i find
> the thread that is holding it. This is not a soft lockup & i dont get any
> hung_task_timeout messages.
> Any help is appreciated.

You could turn on lockdep (LOCKDEP_SUPPORT=y),
and see if any warnings are triggered.

thanks,
Daniel.