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

Dilip Chhetri dchhetri at
Mon Sep 29 21:43:25 UTC 2008

Tijl Coosemans wrote:
> 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:
>>>>  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
>>>>  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
>>>>  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
>>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
> 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.
thanks. I will give that a try, maybe it will work 90% of the time for 
us. Thats much better than having nothing or something that is too 
complicated to implement.
   Thanks once again.

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