From: Lukas Wunner <lukas@wunner.de>
To: Jakub Kicinski <kuba@kernel.org>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>,
Jozsef Kadlecsik <kadlec@netfilter.org>,
Florian Westphal <fw@strlen.de>,
netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
netdev@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@kernel.org>,
Eric Dumazet <edumazet@google.com>, Thomas Graf <tgraf@suug.ch>,
Laura Garcia Liebana <nevola@gmail.com>,
John Fastabend <john.fastabend@gmail.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH nf-next v4 1/5] net: sched: Micro-optimize egress handling
Date: Sun, 24 Jan 2021 11:46:05 +0100 [thread overview]
Message-ID: <20210124104605.GB1056@wunner.de> (raw)
In-Reply-To: <20210123192624.4cee3b7d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
On Sat, Jan 23, 2021 at 07:26:24PM -0800, Jakub Kicinski wrote:
> On Fri, 22 Jan 2021 09:47:01 +0100 Lukas Wunner wrote:
> > sch_handle_egress() returns either the skb or NULL to signal to its
> > caller __dev_queue_xmit() whether a packet should continue to be
> > processed.
> >
> > The skb is always non-NULL, otherwise __dev_queue_xmit() would hit a
> > NULL pointer deref right at its top.
> >
> > But the compiler doesn't know that. So if sch_handle_egress() signals
> > success by returning the skb, the "if (!skb) goto out;" statement
> > results in a gratuitous NULL pointer check in the Assembler output.
>
> Which exact compiler are we talking about it? Did you report this?
> As Eric pointed the compiler should be able to figure this out quite
> easily.
I tested with gcc 8, 9, 10.
No need to report as it's the expected behavior with
-fno-delete-null-pointer-checks, whose motivation appears
questionable though (per my preceding e-mail).
Thanks,
Lukas
prev parent reply other threads:[~2021-01-24 10:47 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1611304190.git.lukas@wunner.de>
[not found] ` <a2a8af1622dff2bfd51d446aa8da2c1d2f6f543c.1611304190.git.lukas@wunner.de>
2021-01-24 3:26 ` Jakub Kicinski
2021-01-24 10:46 ` Lukas Wunner [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210124104605.GB1056@wunner.de \
--to=lukas@wunner.de \
--cc=ast@kernel.org \
--cc=coreteam@netfilter.org \
--cc=daniel@iogearbox.net \
--cc=edumazet@google.com \
--cc=fw@strlen.de \
--cc=john.fastabend@gmail.com \
--cc=kadlec@netfilter.org \
--cc=kuba@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=netfilter-devel@vger.kernel.org \
--cc=nevola@gmail.com \
--cc=pablo@netfilter.org \
--cc=tgraf@suug.ch \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox
all inboxes | Powered by JetHome®