Date:   Tue, 16 Apr 2019 15:44:44 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Arend Van Spriel <arend.vanspriel@broadcom.com>
Cc:     Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= <toke@redhat.com>,
        Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org,
        johannes@sipsolutions.net
Subject: Re: [PATCH 5/5] mac80211: set NETIF_F_LLTX when using intermediate
 tx queues
Message-ID: <20190416074444.pdubbh6fbibdnhi7@gondor.apana.org.au>
References: <20190316170634.13125-1-nbd@nbd.name>
 <20190316170634.13125-5-nbd@nbd.name>
 <87sgvmvg9g.fsf@toke.dk>
 <773e4dff-29fd-22b4-e4bc-cd5a94c66dc2@broadcom.com>
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On Sun, Apr 14, 2019 at 11:44:17AM +0200, Arend Van Spriel wrote:
> + Herbert
> 
> On 3/16/2019 7:14 PM, Toke H?iland-J?rgensen wrote:
> > Felix Fietkau <nbd@nbd.name> writes:
> > 
> > > When using iTXQ, tx sequence number allocation and statistics are run at
> > > dequeue time. Because of that, it is safe to enable NETIF_F_LLTX, which
> > > allows tx handlers to run on multiple CPUs in parallel.
> > 
> > Cool, didn't know about that flag.
> 
> It is water under the bridge as this patch got applied already, but I
> stumbled upon it just recently and didn't know about that flag either. So I
> looked for more information about it and found the definition [1], but the
> comment seemed important enough to send this reply.
> 
> 	NETIF_F_LLTX_BIT,	/* LockLess TX - deprecated. Please */
> 				/* do not use LLTX in new drivers */

The most obvious problem with LLTX is that it can cause AF_PACKET
to see packets twice.  But the more subtle issue is that with a
proper design this lock should have no contention anyway.

So please explain why you want to use this in your driver and where
the contention is coming from?

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt