linux-kernel - Re: [PATCH 1/3] rhashtable: use cmpxchg() in nested_table

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Message-ID: <20190315051029.waqb3ohztl7w6zep@gondor.apana.org.au>
Date:   Fri, 15 Mar 2019 13:10:29 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     NeilBrown <neilb@...e.com>
Cc:     Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] rhashtable: use cmpxchg() in nested_table_alloc()

Hi Neil:

On Thu, Mar 14, 2019 at 04:05:28PM +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.  We can instead protect against
> a race using cmpxchg() - if it the cmp fails, free the table
> that was just allocated.
> 
> Signed-off-by: NeilBrown <neilb@...e.com>
> ---
>  lib/rhashtable.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)

You probably explained it to me before but it's been long enough
that I no longer remember why we need this change.  So please
explain in the commit log why this change is needed.  Because
on the face of it you are adding locking/sychronisation and not
taking it away.

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