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From: NeilBrown <neilb@suse.com>
To: David Miller <davem@davemloft.net>
Cc: tgraf@suug.ch, herbert@gondor.apana.org.au,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects.
Date: Wed, 28 Mar 2018 08:50:31 +1100	[thread overview]
Message-ID: <87sh8l428o.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <20180327.114941.997071660018188736.davem@davemloft.net>

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On Tue, Mar 27 2018, David Miller wrote:

> From: NeilBrown <neilb@suse.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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  parent reply	other threads:[~2018-03-27 21:50 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-26 23:33 [PATCH 0/6] rhashtable: assorted fixes and enhancements NeilBrown
2018-03-26 23:33 ` [PATCH 4/6] rhashtable: allow a walk of the hash table without missing objects NeilBrown
2018-03-27 15:49   ` David Miller
2018-03-27 15:54     ` Herbert Xu
2018-03-27 21:50     ` NeilBrown [this message]
2018-03-27 15:51   ` Herbert Xu
2018-03-27 21:54     ` NeilBrown
2018-03-28  6:07       ` Herbert Xu
2018-03-28  7:17         ` NeilBrown
2018-03-28  7:30           ` Herbert Xu
2018-03-28 21:34             ` NeilBrown
2018-03-29  1:13               ` NeilBrown
2018-03-26 23:33 ` [PATCH 2/6] rhashtable: remove outdated comments about grow_decision etc NeilBrown
2018-03-26 23:33 ` [PATCH 5/6] rhashtable: support guaranteed successful insertion NeilBrown
2018-03-27 15:56   ` Herbert Xu
2018-03-27 21:34     ` NeilBrown
2018-03-28  6:04       ` Herbert Xu
2018-03-28  7:04         ` NeilBrown
2018-03-28  7:27           ` Herbert Xu
2018-03-28 21:26             ` NeilBrown
2018-03-29  5:22               ` Herbert Xu
2018-04-06  3:11                 ` NeilBrown
2018-04-06  4:13                   ` Herbert Xu
2018-03-26 23:33 ` [PATCH 6/6] rhashtable: allow element counting to be disabled NeilBrown
2018-03-26 23:33 ` [PATCH 1/6] rhashtable: improve documentation for rhashtable_walk_peek() NeilBrown
2018-03-27 10:55   ` Sergei Shtylyov
2018-03-27 15:30   ` Herbert Xu
2018-03-27 15:47   ` David Miller
2018-03-27 21:45   ` [PATCH 1/6 v2] " NeilBrown
2018-03-27 22:46   ` [PATCH 1/6] " Andreas Grünbacher
2018-03-28  0:49     ` NeilBrown
2018-03-26 23:33 ` [PATCH 3/6] rhashtable: reset intr when rhashtable_walk_start sees new table NeilBrown
2018-03-27 15:47   ` Herbert Xu

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