Re: [Libunwind-devel] Problem with using libunwind in a ptrace setting where tracer uses pthread

[hai huang]

I found my problem. It's actually not related to libunwind, but how my program is using ptrace and waiting for signals. Thanks!

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Hai

On Wed, Jul 26, 2017 at 10:53 AM, hai huang <address@hidden> wrote:

Hi folks,

I've been using libunwind in an x86_64 Linux environment to do stack walking on ptraced processes with great success. However, that's only when the tracee is single threaded. When the tracee is multi-threaded, I rewrote my tracer program to be multi-threaded using pthread, so that I have a tracer thread for each tracee thread. Each tracer thread has its own libunwind "context", i.e., each thread creates its own libunwind variables as follow:

as = unw_create_addr_space(&_UPT_accessors, 0);
ui = _UPT_create(child);
unw_init_remote(&c, as, ui);

Subsequently, each tracer thread makes unw_step(&c) calls to walk the stack frames of its corresponding tracee thread.

Once I starts the tracer program, I immediately (within a second) see a failure of the call `unw_init_remote(&c, as, ui)`, which returned `-3`. This corresponds to the UNW_EBADREG error.

I'm suspecting I needed some synchronization between my tracer threads as some libunwind states are not multi-threading safe, but I haven't found what exactly I need to protect. Any hints/suspicions would be greatly appreciated! Thanks!

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Hai