netdev - Re: xfrm_state locking regression...

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Date:	Tue, 23 Sep 2008 16:01:29 +0300
From:	Timo Teräs <timo.teras@....fi>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: xfrm_state locking regression...

Herbert Xu wrote:
> On Tue, Sep 23, 2008 at 03:25:41PM +0300, Timo Teräs wrote:
>> There can be a lot of xfrm_state structs. As in thousands. So it
>> will take more memory then. hlist would be enough, so it'd be a
>> 4/8 bytes addition. Single pointer would not be enough as we can
>> have multiple walks on the same entry.
> 
> Right, we need doubly linked lists in the walkers to ease unlinking.
> But only a single pointer (i.e., an hlist) is needed in xfrm_state.

Right.

>> I was thinking about the same. That's why I wasn't so sure if this is
>> better. In practice there aren't many walks active. Maybe we limit
>> the maximum simultaneous walks?
> 
> That sounds like a hack :)

Yes :)

>> But in real life, there is only a one or two walkers if any active.
>> So I would not consider a global update too expensive. But if you think
>> it might become a problem, I'd add an hlist to xfrm_state of walkers
>> referencing that entry.
> 
> Well in real life dumps don't tend to get suspended either :)
> In any case, a suspended dump is only going to pin down some
> memory, while a very long list walk may kill the box.

So, what to do?
1. Go back to: list_del_rcu, xfrm_state_hold(all.next) on delete and
   xfrm_state_put(all.next) on destruct.
2. Add per-entry hlist of walkers currently referencing it.
3. Use the global walker list.

1 can keep memory allocated until userland wakes up. 2 & 3 can make
the delete of that entry slow if there's many walkers suspended.

Btw. the current stuff in net-next is broken. There's no locking
for xfrm_state_walkers list handling.

Cheers,
  Timo
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