Kernel Lock Debugging

On Wed, Aug 31, 2011 at 2:17 PM, Daniel Baluta [off-list ref]wrote:

Hello,

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i wrote a linux kernel module which uses mmap_sem in lots of places. I

grab

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mmap_sem for both reads & writes.  And, i also use page_table_lock

spinlock

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under them. The problem i have is that there is a hard to reproduce a

race

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condition bug in which a thread doesn't release the semaphore at all.

Hence,

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ps hangs as ps needs to grab mmap_sem for the processes. The question is

how

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do i find the thread that is holding the semaphore forever. It will help

me

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figure out the part of the code the race is in.
So, the question is : I have some number of threads grabbing the

semaphore

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for read/write. One of them grabs it & doesn't release at all. How do i

find

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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.

LOCKDEP_SUPPORT is turned on in my kernel but I dont see any warnings.

Thanks though.

Venkatram

thanks,
Daniel.

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