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    SubjectRe: [PATCH] ptrace: add ability to get clear_tid_address
    On Tue, Feb 07, 2012 at 09:15:07PM +0000, Pedro Alves wrote:
    > On 02/07/2012 08:56 PM, Cyrill Gorcunov wrote:
    > > On Tue, Feb 07, 2012 at 08:07:38PM +0000, Pedro Alves wrote:
    > >> On 02/03/2012 04:51 PM, Oleg Nesterov wrote:
    > >>> On 02/03, Pavel Emelyanov wrote:
    > >>
    > >>>> Because there's no need for current to get this value of himself, but can
    > >>>> be useful for e.g. gdb.
    > >>>
    > >>> OK, perhaps this makes sense, I do not know.
    > >>>
    > >>> Jan, Pedro, do you think gdb can use PTRACE_GET_TID_ADDRESS (returns
    > >>> tracee->clear_child_tid) ?
    > >>
    > >> Off hand, I'm not picturing a use. But that may well just mean I'm lacking
    > >> imagination. Andrew, Pavel, did you have a particular idea in mind when you
    > >> said it may be useful for debugging a multithreading program / gdb?
    > >>
    > >
    > > Might not we set up hw watchpoint on this address and get interrupt
    > > before pthread-join will find it? (To be fair I'm not sure if such
    > > trick will work, didn't test ;)
    > For a debugger wanting to know when a pthread_join was about to return?
    > Might be simpler to put a breakpoint (or stap probe, or some such) inside
    > pthread_join.

    Yes, could be, but it means you have to install pthread debug libs, right?
    (have no idea actually since I personally use debug printing instead of

    > It's the kernel that writes to this address, so I've no
    > idea if the watchpoint trap ends up visible on userspace. Which thread
    > would it be reported to, given that this is cleared when the child
    > is gone, I have no idea either.

    Yeah, need some help from someone who wrote hw-breakpoints support in
    kernel (i don't remember the details).


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