Date: Thu, 26 Sep 2013 17:32:08 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Andi Kleen <andi@firstfloor.org>, Lin Ming <minggr@gmail.com>,
        Fengguang Wu <fengguang.wu@intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>, lkp@01.org,
        Tejun Heo <tj@kernel.org>
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers
 into n_tty_data"
Message-ID: <20130926153208.GN18242@two.firstfloor.org>
References: <20130913005133.GA32479@localhost>
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 <5238767D.1080606@hurleysoftware.com>
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 <CAF1ivSbYg296oHhO0_UXwkos8Ui_1C93pUnXPzow4qLWyAa2kg@mail.gmail.com>
 <5242C960.7050506@hurleysoftware.com>
 <1380124944.27676.1.camel@monkey32>
 <87r4cc8e0q.fsf@tassilo.jf.intel.com>
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On Thu, Sep 26, 2013 at 07:52:23AM -0400, Peter Hurley wrote:
> On 09/25/2013 11:20 PM, Andi Kleen wrote:
> >Lin Ming <minggr@gmail.com> writes:
> >>
> >>Would you like below patch?
> >
> >The loop body keeps rather complex state. It could easily
> >get confused by parallel RCU changes.
> >
> >So if the list changes in parallel you may suddenly
> >report very bogus values, as the va_start - prev_end
> >computation may be bogus.
> >
> >Perhaps it's ok (may report bogus gaps), but it seems a bit risky.
> 
> I don't understand how the computed gap would be bogus; there
> _was_ a list state in which that particular gap existed. The fact

It could change any time as you don't have an atomic view
of vm_end / vm_start. It is valid to change the fields
with the lock held.

-Andi
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