linux-kernel - Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty

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Message-ID: <20130926153208.GN18242@two.firstfloor.org>
Date:	Thu, 26 Sep 2013 17:32:08 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Lin Ming <minggr@...il.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org,
	Tejun Heo <tj@...nel.org>
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers
 into n_tty_data"

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@...il.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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