netdev - Re: [Patch net-next v3] net_sched: move the empty tp check from ->destroy() to ->delete()

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Date:   Tue, 18 Apr 2017 08:51:11 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Lucas Bates <lucasb@...atatu.com>
Subject: Re: [Patch net-next v3] net_sched: move the empty tp check from
 ->destroy() to ->delete()

On 17-04-17 02:30 PM, Cong Wang wrote:
> Roi reported we could have a race condition where in ->classify() path
> we dereference tp->root and meanwhile a parallel ->destroy() makes it
> a NULL.
>
> This is possible because ->destroy() could be called when deleting
> a filter to check if we are the last one in tp, this tp is still
> linked and visible at that time.
>
> Daniel fixed this in commit d936377414fa
> ("net, sched: respect rcu grace period on cls destruction"), but
> the root cause of this problem is the semantic of ->destroy(), it
> does two things (for non-force case):
>
> 1) check if tp is empty
> 2) if tp is empty we could really destroy it
>
> and its caller, if cares, needs to check its return value to see if
> it is really destroyed. Therefore we can't unlink tp unless we know
> it is empty.
>
> As suggested by Daniel, we could actually move the test logic to ->delete()
> so that we can safely unlink tp after ->delete() tells us the last one is
> just deleted and before ->destroy().
>
> What's more, even we unlink it before ->destroy(), it could still have
> readers since we don't wait for a grace period here, we should not modify
> tp->root in ->destroy() either.
>
> Reported-by: Roi Dayan <roid@...lanox.com>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: John Fastabend <john.fastabend@...il.com>
> Cc: Jamal Hadi Salim <jhs@...atatu.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

Looks good to me, Daniel?

Also - if we are not in a hurry could I/Lucas (and maybe Roi) be given 
the  chance to test it?

cheers,
jamal