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Date: Thu, 19 Dec 2013 19:16:06 +0400
From: Gleb Smirnoff <glebius@FreeBSD.org>
To: Adrian Chadd <adrian@freebsd.org>
Subject: Re: Removing queue length check in ip_output (was Re: buf_ring in
 HEAD is racy)
Message-ID: <20131219151606.GB71033@FreeBSD.org>
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Cc: Jack F Vogel <jfv@freebsd.org>,
 Michael Tuexen <Michael.Tuexen@lurchi.franken.de>,
 Ryan Stone <rysto32@gmail.com>, freebsd-net <freebsd-net@freebsd.org>
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On Wed, Dec 18, 2013 at 01:04:43PM -0800, Adrian Chadd wrote:
A> How about we can this check in ip_output():
A> 
A>         /*
A>          * Verify that we have any chance at all of being able to queue the
A>          * packet or packet fragments, unless ALTQ is enabled on the given
A>          * interface in which case packetdrop should be done by queueing.
A>          */
A>         n = ip_len / mtu + 1; /* how many fragments ? */
A>         if (
A> #ifdef ALTQ
A>             (!ALTQ_IS_ENABLED(&ifp->if_snd)) &&
A> #endif /* ALTQ */
A>             (ifp->if_snd.ifq_len + n) >= ifp->if_snd.ifq_maxlen ) {
A>                 error = ENOBUFS;
A>                 IPSTAT_INC(ips_odropped);
A>                 ifp->if_snd.ifq_drops += n;
A>                 goto bad;
A>         }
A> 
A> .. it's totally bogus in an if_transmit / SMP world. There's no
A> locking and there's no guarantee that there will be headroom in the
A> queue between this point and the later call to the if_output() method.

My vote is to remove this. But better leave a comment that we need
an API to send a bunch of packets (or fail them all) via an interface.

-- 
Totus tuus, Glebius.