linux-kernel - Re: [PATCH v2] netlink: Replace rhash

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Message-ID: <CA+55aFyrctX5UH7Y+0731Tmb-HzjxxFt-c2cv-4iG9aCZPmsaw@mail.gmail.com>
Date:	Tue, 22 Sep 2015 14:04:33 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	Tejun Heo <tj@...nel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>,
	Cong Wang <cwang@...pensource.com>,
	Tom Herbert <tom@...bertland.com>, kafai@...com,
	kernel-team@...com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Jiří Pírko <jiri@...nulli.us>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Thomas Graf <tgraf@...g.ch>, Scott Feldman <sfeldma@...il.com>
Subject: Re: [PATCH v2] netlink: Replace rhash_portid with bound

On Tue, Sep 22, 2015 at 1:36 PM, Bjørn Mork <bjorn@...k.no> wrote:
>
> http://download.intel.com/design/archives/processors/pro/docs/24268935.pdf
>
> Says "NoFix" for erratas 66 and 92.

Yeah, 66 and 92 do look like they could cause the apparent ordering of
accesses to be violated. That said, both of them <i>seem</i> to be
"processor had exclusive access to line A, and gave it away but ended
up still reading now-stale data".

And that's not what we use "smp_wmb()" or "smp_rmb()" to protect
against. If we did a write and then wanted to do an ordered read, we'd
use smp_mb(), which always does that barrier.

So I don't know whether either of those really merit our PPRO
workaround. Cache coherency is hard.

There's also errata 41, which looks like it would be a bad situation.

                 Linus
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