linux-kernel - Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects.

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Date:   Wed, 28 Mar 2018 08:50:31 +1100
From:   NeilBrown <neilb@...e.com>
To:     David Miller <davem@...emloft.net>
Cc:     tgraf@...g.ch, herbert@...dor.apana.org.au, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects.

On Tue, Mar 27 2018, David Miller wrote:

> From: NeilBrown <neilb@...e.com>
> Date: Tue, 27 Mar 2018 10:33:04 +1100
>
>> In many cases where the walker needs to drop out of RCU protection,
>> it will take a reference to the object and this can prevent it from
>> being removed from the hash table.  In those cases, the last-returned
>> object can still be used as a cursor.  rhashtable cannot detect
>> these cases itself.
>
> Merely having an elevated reference count does not explicitly prevent
> the object from being removed from the hash table.
>
> This invariant might hold for the particular user of the rhashtable
> instance, but it is not always the case.

Agreed.  Hence "In many case ... this *can* be prevented" and "In those
cases".

The doc comment for rhashtable_walk_start_continue() makes it clear
that:

 *   The
 * previously returned object must still be in the hash table, and must be
 * provided as an argument.

Thanks,
NeilBrown

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