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

From: Jon Maloy
Date: Wed Jul 10 2019 - 09:10:28 EST

> -----Original Message-----
> From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Sent: 10-Jul-19 04:00
> To: Jon Maloy <jon.maloy@xxxxxxxxxxxx>; Eric Dumazet
> <eric.dumazet@xxxxxxxxx>; Chris Packham
> <Chris.Packham@xxxxxxxxxxxxxxxxxxx>; ying.xue@xxxxxxxxxxxxx;
> davem@xxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx; tipc-discussion@xxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> 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.