Re: [patch 1/4] Add routine for generating an ID for kernel pointer

From: Cyrill Gorcunov
Date: Wed Dec 28 2011 - 14:48:20 EST

On Wed, Dec 28, 2011 at 09:35:02PM +0400, Cyrill Gorcunov wrote:
> On Wed, Dec 28, 2011 at 05:21:51PM +0000, Alan Cox wrote:
> > > It can happen in case of object re-allocated from slab. But in case
> > > of two living pids it's impossible to get same pointers for different
> > > objects. Or I misunderstood the question, Alan? It's up to application
> > > to not compare objects from dead tasks.
> >
> > How will it know the task has not died and been reallocated, or its
> > resources not been freed and reallocated during the comparison ?
> >
> > Your comparison appears to have a zero time validity - you can ask "is A
> > the same as B" but by the time you get an answer your answer may no
> > longer be true. It also externalises a current implementation
> > detail in a very ugly way.
> >
> It's not differ from reading other data from /proc. How make you be sure
> the PPid read from a task status is the same once you've finished reading
> it? The same applies to say ps output, it's valid for almost zero time,
> then one need to restart ps again to get new process tree snapshot. The
> same here -- if I need a precise results I have to either stop tasks or
> froze them and compare IDs. That's what I've had in mind.
> > Would it also not be better to do the job right and simply have an
> > interface to ask "who shares with A" or even something a bit more high
> > level, it seems you are creating something nastier by trying to push all
> > this in userspace than if you did the job or part of it kernel side where
> > you had access to the right locking infrastructure and where the public
> > API doesn't need to expose innards of the kernel ?
> >
> Need to think, thanks a lot for idea, Alan!

You know, Alan, I thought about different ways... but eventually I think
having _this_ interface is better than anything else (of course along with
addressing problems Tejun mentioned). "Who shares with A" doesn't make
situation better, once I obtain the result it's not valid anymore and
I have to either do the same requests again and again, or stop/freeze
tasks before asking which resources they do share. The interface provided
now gives the *minimun* information user-space needed to draw the shared
resources affinity scheme, and I would like to escape putting more work
on kernel side if possible. Sounds reasonable?

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