linux-kernel - Re: [PATCH 1/4] rhashtable: use cmpxchg() in nested_table

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Message-ID: <20190408023445.ihid3a6tx6k37xhd@gondor.apana.org.au>
Date:   Mon, 8 Apr 2019 10:34:46 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     NeilBrown <neilb@...e.com>
Cc:     Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] rhashtable: use cmpxchg() in nested_table_alloc()

On Tue, Apr 02, 2019 at 10:07:45AM +1100, NeilBrown wrote:
> nested_table_alloc() relies on the fact that there is
> at most one spinlock allocated for every slot in the top
> level nested table, so it is not possible for two threads
> to try to allocate the same table at the same time.
> 
> This assumption is a little fragile (it is not explicit) and is
> unnecessary as cmpxchg() can be used instead.
> 
> A future patch will replace the spinlocks by per-bucket bitlocks,
> and then we won't be able to protect the slot pointer with a spinlock.
> 
> So replace rcu_assign_pointer() with cmpxchg() - which has equivalent
> barrier properties.
> If it the cmp fails, free the table that was just allocated.
> 
> Signed-off-by: NeilBrown <neilb@...e.com>

Acked-by: Herbert Xu <herbert@...dor.apana.org.au>
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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