linux-kernel - Re: still nfs problems [Was: Linux 2.6.37-rc8]

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <AANLkTimusF4KCJ4vHPn+T1dUiKxapXaT0QTtFvh8Ykdd@mail.gmail.com>
Date:	Thu, 6 Jan 2011 09:55:30 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Marc Kleine-Budde <mkl@...gutronix.de>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Marc Kleine-Budde <m.kleine-budde@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org,
	Parisc List <linux-parisc@...r.kernel.org>,
	linux-arch@...r.kernel.org
Subject: Re: still nfs problems [Was: Linux 2.6.37-rc8]

On Thu, Jan 6, 2011 at 9:47 AM, Trond Myklebust
<Trond.Myklebust@...app.com> wrote:
>
> Why is this line needed? We're not writing through the virtual mapping.

I haven't looked at the sequence of accesses, but you need to be
_very_ aware that "write-through" is absolutely NOT sufficient for
cache coherency.

In cache coherency, you have three options:

 - true coherency (eg physically indexed/tagged caches)

 - exclusion (eg virtual caches, but with an exclusion guarantee that
guarantees that aliases cannot happen: either by using physical
tagging or by not allowing cases that could cause virtual aliases)

 - write-through AND non-cached reads (ie "no caching at all").

You seem to be forgetting the "no cached reads" part. It's not
sufficient to flush after a write - you need to make sure that you
also don't have a cached copy of the alias for the read.

So "We're not writing through the virtual mapping" is NOT a sufficient
excuse. If you're reading through the virtual mapping, you need to
make sure that the virtual mapping is flushed _after_ any writes
through any other mapping and _before_ any reads through the virtual
one.

This is why you really really really generally don't want to have
aliasing. Purely virtual caches are pure crap. Really.

                       Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/