linux-kernel - Re: [PATCH] ptrace: add ability to get clear_tid

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Date:	Wed, 8 Feb 2012 16:42:54 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Pedro Alves <palves@...hat.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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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