linux-kernel - RE: [PATCH] tipc: ensure skb->lock is initialised

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Message-ID: <CH2PR15MB357528CF6DC0A587D2CB90689AF30@CH2PR15MB3575.namprd15.prod.outlook.com>
Date:   Thu, 11 Jul 2019 12:55:12 +0000
From:   Jon Maloy <jon.maloy@...csson.com>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "ying.xue@...driver.com" <ying.xue@...driver.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] tipc: ensure skb->lock is initialised



> -----Original Message-----
> From: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
> Sent: 10-Jul-19 16:58
> To: Jon Maloy <jon.maloy@...csson.com>; Eric Dumazet
> <eric.dumazet@...il.com>; ying.xue@...driver.com;
> davem@...emloft.net
> Cc: netdev@...r.kernel.org; tipc-discussion@...ts.sourceforge.net; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH] tipc: ensure skb->lock is initialised
> 
> 
> On 11/07/19 1:10 AM, Jon Maloy wrote:
> >> -----Original Message-----
> >> From: Eric Dumazet <eric.dumazet@...il.com>
> >> Sent: 10-Jul-19 04:00
> >> To: Jon Maloy <jon.maloy@...csson.com>; Eric Dumazet
> >> <eric.dumazet@...il.com>; Chris Packham
> >> <Chris.Packham@...iedtelesis.co.nz>; ying.xue@...driver.com;
> >> davem@...emloft.net
> >> Cc: netdev@...r.kernel.org; tipc-discussion@...ts.sourceforge.net;
> >> linux- kernel@...r.kernel.org
> >> Subject: Re: [PATCH] tipc: ensure skb->lock is initialised
> >>
> >>
> >>
> >> On 7/9/19 10:15 PM, Jon Maloy wrote:
> >>>
> >>> It is not only for lockdep purposes, -it is essential.  But please
> >>> provide details
> >> about where you see that more fixes are needed.
> >>>
> >>
> >> Simple fact that you detect a problem only when skb_queue_purge() is
> >> called should talk by itself.
> >>
> >> As I stated, there are many places where the list is manipulated
> >> _without_ its spinlock being held.
> >
> > Yes, and that is the way it should be on the send path.
> >
> >>
> >> You want consistency, then
> >>
> >> - grab the spinlock all the time.
> >> - Or do not ever use it.
> >
> > That is exactly what we are doing.
> > - The send path doesn't need the spinlock, and never grabs it.
> > - The receive path does need it, and always grabs it.
> >
> > However, since we don't know from the beginning which path a created
> > message will follow, we initialize the queue spinlock "just in case"
> > when it is created, even though it may never be used later.
> > You can see this as a violation of the principle you are stating
> > above, but it is a prize that is worth paying, given savings in code
> > volume, complexity and performance.
> >
> >>
> >> Do not initialize the spinlock just in case a path will use
> >> skb_queue_purge() (instead of using __skb_queue_purge())
> >
> > I am ok with that. I think we can agree that Chris goes for that
> > solution, so we can get this bug fixed.
> 
> So would you like a v2 with an improved commit message? I note that I said
> skb->lock instead of head->lock in the subject line so at least that should be
> corrected.

Yes, unless Eric has any more objections. 
I addition, I have realized that we can change from skb_queue_head_init()  to __skb_queue_head_init() at all the disputed locations in the socket code.
Then, we do a separate call to spin_lock_init(&list->lock) at the moment we find that the message will follow the receive path, i.e., in tipc_node_xmit().
That should make everybody happy. 
I will post a patch when net-next re-opens.

BR
///jon