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SubjectRe: [PATCH] ptrace: add ability to get clear_tid_address
On Wed, Feb 08, 2012 at 12:30:47PM +0000, Pedro Alves wrote:
> >
> > 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
> > breakpoints).
>
> Not really more than what we need today. Just some exported function name
> in the elf symbol tables. Assuming a the program is using clear_child_tid
> address for pthread_join as glibc is may not be a good idea. It's doing
> things at the wrong layer. Also, hardware watchpoints are a scarse
> resource.

It's pretty precious resource still incredibly useful. ok, i see
what you mean, thanks.

>
> >> 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).
>
> I just tried it. This is &pthread->tid in glibc/libpthread, so with debug
> info it's easy to figure out where to set the watchpoint manually with gdb
> without asking the kernel. Doesn't work. ptrace doesn't show any trap
> for the kernel writes.
>

Thanks for info.

Cyrill


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