netdev - Re: Potential race in ip4_datagram_release

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Date:	Fri, 6 Jun 2014 11:13:54 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexey Preobrazhensky <preobr@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Lars Bull <larsbull@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Bruce Curtis <brutus@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	dormando <dormando@...ia.net>
Subject: Re: Potential race in ip4_datagram_release_cb

On Fri, Jun 6, 2014 at 10:56 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2014-06-06 at 10:44 -0700, Alexei Starovoitov wrote:
>> On Fri, Jun 6, 2014 at 9:16 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>> > We should either :
>> >
>> > 1) use xchg() and no lock at all to change sk_dst_cache, as we did for
>> > sk_rx_dst ( cf udp_sk_rx_dst_set() )
>> >
>> > 2) No longer use sk_dst_lock, and always use the socket lock
>> > (sk->sk_lock.slock) instead.
>>
>> Probably needs some combination of both.
>
> I do not think so.
>
> If we use xchg(), then we do not need anything else.
>
> If we use a spinlock, then xchg() seems useless.
>
> Any combination is buggy.
>
> In the past, UDP transmit was holding socket lock,
> but we added a lockless path, and I suppose more people
> use a UDP socket from different threads.
>
> Then, Steffen added the ip4_datagram_release_cb() thing,
> increasing the chance of mixing both 'protections'.
>
> Setting sk_dst_cache should hardly be a hot path.

yeah. if we use sk_lock.slock then xchg is obviously not needed.
By 'both' I meant to do xchg() and get rid of sk_dst_lock to speed it up.
Though not worth going too fancy if speed is not needed.

>
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