linux-kernel - Re: [PATCH 3/4] rhashtable: use bit_spin

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Message-ID: <20190326050320.gwk3tgtqwl5csivt@gondor.apana.org.au>
Date:   Tue, 26 Mar 2019 13:03:20 +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,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH 3/4] rhashtable: use bit_spin_locks to protect hash
 bucket.

On Mon, Mar 25, 2019 at 04:05:39PM +1100, NeilBrown wrote:
>
> + * Sometimes we unlock a bucket by writing a new pointer there.  In that
> + * case we don't need to unlock, but we do need to reset state such as
> + * local_bh. For that we have rht_unlocked().  This doesn't include
> + * the memory barrier that bit_spin_unlock() provides, but rcu_assign_pointer()
> + * will have provided that.

Hmm, are you sure that's enough? IIRC rcu_assign_pointer only
provides a write barrier compared to the more complete (but one-way)
barrier that a spin-lock provides.

Cheers,
-- 
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