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

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Date:	Tue, 07 Feb 2012 21:15:07 +0000
From:	Pedro Alves <palves@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
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 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.

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.

-- 
Pedro Alves
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