getting stack trace for other thread on the same process : libthr

Tijl Coosemans tijl at
Mon Sep 29 20:09:57 UTC 2008

On Monday 29 September 2008 18:59:36 Dilip Chhetri wrote:
> Tijl Coosemans wrote:
>> On Friday 26 September 2008 21:06:58 Dilip Chhetri wrote:
>>> Question
>>> --------
>>>   My program is linked with libthr in FreeBSD-7.0. The program has
>>> in the order of 20 threads, and a designated monitoring thread at
>>> some point wants to know what are other/stuck threads doing. This
>>> needs to be done by printing stack backtrace for the thread to
>>> stdout.
>>>   I understand pthread_t structure has pointer to the target
>>> thread's stack, but to get the trace I need to know value of
>>> stack-pointer register and base-pointer register. I looked at the
>>> code and I don't find any mechanism by which I could read the
>>> target threads register context (because it all resides within
>>> kernel thread structure). Further code study reveals that
>>> kernel_thread->td_frame contains the register context for a thread,
>>> but is valid only when the thread is executing/sleeping inside the
>>> kernel.
>>>   Is there anything I'm missing here ? Is there an easy way to
>>> traverse stack for some thread with in the same process.
>>>   I considered/considering following approaches,
>>> a) use PTRACE
>>>    ruled out, because you can't trace the process from within the
>>>    same process
>>> b) somehow temporarily stop the target-thread and read td_frame by
>>>    traversing kernel data structure through /dev/kmem. After doing
>>>    stack traversal resume the target thread.
>>> Detailed problem background
>>> --------------------------
>>>   We have this process X with ~20 threads, each processing some
>>> requests. One of them is designated as monitoring/dispatcher
>>> thread. When a new request arrives, dispatcher thread tries to
>>> queue the task to idle thread. But if all threads are busy
>>> processing requests, the dispatcher thread is supposed to print the
>>> stack back trace for each of the busy thread. This is our
>>> *debugging* mechanism to find potential fault-points.
>>>   In FreeBSD-4.6.2, we hacked libc_r:pthread_t to achieve our goal.
>>> But in FreeBSD-7.0, we decided to use libthr and hack doesn't seem
>>> to be easy.
>>> Target setup
>>> ------------
>>>  * SMP     : around 8 CPU
>>>  * process : it's going to be run as root and have around ~20
>>>  threads 
>> You could try registering a signal handler for SIGUSR1 that prints a
>> stack backtrace using the stack pointer in the sigcontext and then
>> call pthread_kill(SIGUSR1) on whichever thread you want a backtrace
>> of.
> Thanks, but as I mentioned it's a network based program and it may be
> sleeping/stuck in syscall for some packets, in this case pthread_kill
> will not work because signals are delivered only when you return from
> syscall (that's what I haved learned from old UNIX books in my
> college).

Those kind of syscalls are usually interruptable though. Depending on
the SA_RESTART flag they are then either aborted and return EINTR or
restarted (or return partial success). See the sigaction(2) manpage.

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