Date: Mon, 10 Mar 2003 12:11:20 -0800 (PST) From: Nate Lawson <nate@root.org> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: cvs commit: src/sys/sys devicestat.h Message-ID: <Pine.BSF.4.21.0303101159120.93916-100000@root.org> In-Reply-To: <2986.1047320558@critter.freebsd.dk>
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On Mon, 10 Mar 2003, Poul-Henning Kamp wrote:
> In message <Pine.BSF.4.21.0303101009420.93727-100000@root.org>, Nate Lawson wri
> tes:
>
> Doesn't work:
>
> CPU0 (kernel) CPU1 (userland)
>
> increment seq
> start copy
> fiddle data
> end copy
> (copy is inconsistent)
Please see my original message, including the code fragment. You
neglected the final step which was:
(userland)
check localbuf->seq == kernbuf->seq
Of course leaving out steps of my approach breaks it. :)
Msg: <Pine.BSF.4.21.0303100924470.93706-100000@root.org>
> Notice that to make it absolutely water-tight, the seq fields must
> be updated with appropriate write barriers and the order of the
> fields must be the opposite as the direction the copy is performed in.
Agree but with one sequence checked twice (pulled in as first part of
memcpy, explicitly checked again after the copy is complete) you get this
for free without worrying about the order of updating two sequences. The
only downside to my approach is that it could mark data as invalid if the
seq is updated between the memcpy and the seq check. But it will never
mark invalid data as valid.
> I may not think it worth going to that effort for io stats.
ken@ should probably have the final say with what goes for devicestat. It
doesn't seem like you are interested in considering alternate approaches
and it is such a small area that I don't feel like debating it endlessly.
-Nate
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