linux-kernel - Re: rhashtable: Prevent spurious EBUSY errors on insertion

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Message-Id: <20151204.143844.1840501199234860407.davem@davemloft.net>
Date:	Fri, 04 Dec 2015 14:38:44 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	phil@....cc, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	tgraf@...g.ch, fengguang.wu@...el.com, wfg@...ux.intel.com,
	lkp@...org
Subject: Re: rhashtable: Prevent spurious EBUSY errors on insertion

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Thu, 3 Dec 2015 20:41:29 +0800

> Thomas and Phil observed that under stress rhashtable insertion
> sometimes failed with EBUSY, even though this error should only
> ever been seen when we're under attack and our hash chain length
> has grown to an unacceptable level, even after a rehash.
> 
> It turns out that the logic for detecting whether there is an
> existing rehash is faulty.  In particular, when two threads both
> try to grow the same table at the same time, one of them may see
> the newly grown table and thus erroneously conclude that it had
> been rehashed.  This is what leads to the EBUSY error.
> 
> This patch fixes this by remembering the current last table we
> used during insertion so that rhashtable_insert_rehash can detect
> when another thread has also done a resize/rehash.  When this is
> detected we will give up our resize/rehash and simply retry the
> insertion with the new table.
> 
> Reported-by: Thomas Graf <tgraf@...g.ch>
> Reported-by: Phil Sutter <phil@....cc>
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

Looks good, applied, thanks Herbert.
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