netdev - Re: [PATCH] rhashtable: don't attempt to grow when at max

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Date:	Thu, 23 Apr 2015 21:46:28 +0100
From:	Thomas Graf <tgraf@...g.ch>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	netdev@...r.kernel.org, Patrick McHardy <kaber@...sh.net>,
	Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH] rhashtable: don't attempt to grow when at max_size

On 04/23/15 at 04:38pm, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> The conversion of mac80211's station table to rhashtable had a bug
> that I found by accident in code review, that hadn't been found as
> rhashtable apparently managed to have a maximum hash chain length
> of one (!) in all our testing.

This is the desired chain length ;-)

> In order to test the bug and verify the fix I set my rhashtable's
> max_size very low (4) in order to force getting hash collisions.
> 
> At that point, rhashtable WARNed in rhashtable_insert_rehash() but
> didn't actually reject the hash table insertion. This caused it to
> lose insertions - my master list of stations would have 9 entries,
> but the rhashtable only had 5. This may warrant a deeper look, but
> that WARN_ON() just shouldn't happen.

The warning got fixed recently (51bb8e331b) and
rhashtable_insert_rehash() now only allows a single rehash if at
max_size already. It will now return -EBUSY.

Insertions may still fail while the table is above 100% utilization
so this fix is absolutely needed though.

> Fix this by not returning true from rht_grow_above_100() when the
> rhashtable's max_size has been reached - in this case the user is
> explicitly configuring it to be at most that big, so even if it's
> now above 100% it shouldn't attempt to resize.

Good catch. I wonder whether we want to trigger a periodic rehash
in an interval in this situation or just leave this up to the user
to setup a timer himself.

> This fixes the "lost insertion" issue and consequently allows my
> code to display its error (and verify my fix for it.)
> 
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

Acked-by: Thomas Graf <tgraf@...g.ch>
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