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

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Message-ID: <20140124171754.GM31040@mudshark.cambridge.arm.com>
Date:	Fri, 24 Jan 2014 17:17:55 +0000
From:	Will Deacon <will.deacon@....com>
To:	Adrien Vergé <adrienverge@...il.com>
Cc:	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Mathieu Poirier <mathieu.poirier@...aro.org>,
	Christopher Covington <cov@...eaurora.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 4/6] ARM: Make PID_IN_CONTEXTIDR incompatible with
 PID_NS

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.

Will
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