Re: Buggy rhashtable walking

[Herbert Xu]

On Fri, Aug 05, 2016 at 04:46:43AM -0700, Ben Greear wrote:
>
> It would not be fun to have to revert to the old way of hashing
> stations in mac80211...
> 
> I'll be happy to test the patches when you have them ready.

Thanks for the offer.  Unfortunately it'll be a few days before
I'm ready because I need to work through some crypto patches first.

Cheers,
-- 
Email: Herbert Xu 
Home Page: http://gondor.apana.org.au/~herbert/
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Re: Buggy rhashtable walking

[Ben Greear]

On 08/05/2016 03:50 AM, Johannes Berg wrote:

On Fri, 2016-08-05 at 18:48 +0800, Herbert Xu wrote:

On Fri, Aug 05, 2016 at 08:16:53AM +0200, Johannes Berg wrote:


Hm. Would you rather allocate a separate head entry for the
hashtable,
or chain the entries?


My plan is to build support for this directly into rhashtable.
So I'm adding a struct rhlist_head that would be used in place
of rhash_head for these cases and it'll carry an extra pointer
for the list of identical entries.

I will then add an additional layer of insert/lookup interfaces
for rhlist_head.


Herbert, thank you for fixing this!

It would not be fun to have to revert to the old way of hashing
stations in mac80211...

I'll be happy to test the patches when you have them ready.

Ben


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Ben Greear 
Candela Technologies Inc  http://www.candelatech.com
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Re: Buggy rhashtable walking

[Herbert Xu]

On Fri, Aug 05, 2016 at 08:16:53AM +0200, Johannes Berg wrote:
> 
> Hm. Would you rather allocate a separate head entry for the hashtable,
> or chain the entries?

My plan is to build support for this directly into rhashtable.
So I'm adding a struct rhlist_head that would be used in place
of rhash_head for these cases and it'll carry an extra pointer
for the list of identical entries.

I will then add an additional layer of insert/lookup interfaces
for rhlist_head.

So bottom-line is that if you have no identical entries that you
only incur an extra 8 bytes per-object.

Cheers,
-- 
Email: Herbert Xu 
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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Re: Buggy rhashtable walking

[Johannes Berg]

On Fri, 2016-08-05 at 18:48 +0800, Herbert Xu wrote:
> On Fri, Aug 05, 2016 at 08:16:53AM +0200, Johannes Berg wrote:
> > 
> > Hm. Would you rather allocate a separate head entry for the
> > hashtable,
> > or chain the entries?
> 
> My plan is to build support for this directly into rhashtable.
> So I'm adding a struct rhlist_head that would be used in place
> of rhash_head for these cases and it'll carry an extra pointer
> for the list of identical entries.
> 
> I will then add an additional layer of insert/lookup interfaces
> for rhlist_head.

Oh, ok.

> So bottom-line is that if you have no identical entries that you
> only incur an extra 8 bytes per-object.
> 
Right.

Thanks!

johannes
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Re: Buggy rhashtable walking

[Johannes Berg]

> So I'm going to fix this by consolidating identical objects into
> a single rhashtable entry which also lets us get rid of the
> insecure_elasticity setting.

Hm. Would you rather allocate a separate head entry for the hashtable,
or chain the entries?

(Luckily) the colliding key case practically never happens, and some
drivers don't even allow it, so that's perhaps something to keep in
mind for this. Perhaps we should just generally disallow it - but a few
people (hi Ben) would be really unhappy about that I guess.

I think this might affect more than one use of rhashtable in mac80211
now, since the mesh paths also use it.

johannes
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Re: Buggy rhashtable walking

[Herbert Xu]

On Thu, Aug 04, 2016 at 03:18:46PM +0800, Herbert Xu wrote:
> 
> So the question is can wireless handle seeing an entry multiple
> times? In particular, __ieee80211_rx_handle_packet would appear
> to process the same packet multiple times if this were to happen.

It's worse than I thought.  In fact it's not walking the table
at all, rather it's doing a hash lookup by hand!

This cannot possibly work given that rhashtable makes use of
multiple hash tables.

In fact this also demonstrates why putting multiple identical
objects into the same table is crap.  Because there is no sane
way of returning all objects corresponding to a single key, given
that they may be spread over multiple tables.

So I'm going to fix this by consolidating identical objects into
a single rhashtable entry which also lets us get rid of the
insecure_elasticity setting.

So the next time someone comes along and wants to add multiple
objects with the same key to one table, please just say no.

Thanks,
-- 
Email: Herbert Xu 
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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