linux-kernel - Re: [PATCH V2 4/6] ARM: Make PID_IN_CONTEXTIDR incompatible with PID

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Message-ID: <52E2BB09.1020008@codeaurora.org>
Date:	Fri, 24 Jan 2014 14:12:09 -0500
From:	Christopher Covington <cov@...eaurora.org>
To:	Will Deacon <will.deacon@....com>
CC:	Dirk Behme <dirk.behme@...bosch.com>,
	Russell King <linux@....linux.org.uk>,
	Mathieu Poirier <mathieu.poirier@...aro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Randy Dunlap <rdunlap@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Adrien Vergé <adrienverge@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>
Subject: Re: [PATCH V2 4/6] ARM: Make PID_IN_CONTEXTIDR incompatible with
 PID_NS

On 01/24/2014 12:52 PM, Christopher Covington wrote:
> On 01/24/2014 12:17 PM, Will Deacon wrote:
>> On Fri, Jan 24, 2014 at 05:16:28PM +0000, Adrien Vergé wrote:
>>> 2014/1/24 Will Deacon <will.deacon@....com>:
>>>> Are you sure about this? The value we write is actually task_pid_nr, which I
>>>> believe to be globally unique.
>>>
>>> You are right: the task_pid_nr is unique in the system. However when
>>> using namespaces, the so called "PID" is the virtual number that
>>> processes in different namespaces can share.
>>>
>>> This PID is the one visible by user-space tasks, in particular
>>> user-space tracers and debuggers. These programs would expect to find
>>> the PID of the traced process in the Context ID reg, while it is not.
>>> I think it is better to remove confusion by making PID_IN_CONTEXTIDR
>>> and PID_NS incompatible.
>>>
>>> What do you think?
>>
>> I think I'd rather have the global ID than disable a potentially useful
>> feature, especially since this is likely to be consumed by external trace
>> tools as opposed to user-space tasks.
> 
> We've discussed before that the ARM architecture doesn't say what should be
> written to the CONTEXTIDR, so it's up to us to decide. Will has a use case
> where the global PID is useful. Adrien's patches present a use case where I
> think the virtual PID would be useful. I've done work in the past where
> writing the process group ID was useful. Would it be reasonable to make what's
> written to the CONTEXTIDR run-time configurable? If so, what would be the best
> interface for configuring it?

D'oh, I mixed things up. For ETM to work it can only use global PID's in the
CONTEXTIDR.

Christopher

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