From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: [PATCH net-next] rhashtable: further improve stability of rhashtable_walk Date: Mon, 10 Dec 2018 09:50:43 +1100 Message-ID: <87k1kico1o.fsf@notabene.neil.brown.name> References: <153086101070.2825.6850140624411927465.stgit@noble> <153086109256.2825.15329014177598382684.stgit@noble> <87zhtkeimx.fsf@notabene.neil.brown.name> <20181207053943.7zacyn5uvqkfnfoi@gondor.apana.org.au> Mime-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Cc: Thomas Graf , Tom Herbert , David Miller , netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: Herbert Xu Return-path: In-Reply-To: <20181207053943.7zacyn5uvqkfnfoi@gondor.apana.org.au> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On Fri, Dec 07 2018, Herbert Xu wrote: > On Wed, Dec 05, 2018 at 02:51:02PM +1100, NeilBrown wrote: >>=20 >> If the sequence: >> obj =3D rhashtable_walk_next(iter); >> rhashtable_walk_stop(iter); >> rhashtable_remove_fast(ht, &obj->head, params); >> rhashtable_walk_start(iter); >>=20 >> races with another thread inserting or removing >> an object on the same hash chain, a subsequent >> rhashtable_walk_next() is not guaranteed to get the "next" >> object. It is possible that an object could be >> repeated, or missed. >>=20 >> This can be made more reliable by keeping the objects in a hash chain >> sorted by memory address. A subsequent rhashtable_walk_next() >> call can reliably find the correct position in the list, and thus >> find the 'next' object. >>=20 >> It is not possible to take this approach with an rhltable as keeping >> the hash chain in order is not so easy. When the first object with a >> given key is removed, it is replaced in the chain with the next >> object with the same key, and the address of that object may not be >> correctly ordered. >> I have not yet found any way to achieve the same stability >> with rhltables, that doesn't have a major impact on lookup >> or insert. No code currently in Linux would benefit from >> such extra stability. >>=20 >> With this patch: >> - a new object is always inserted after the last object with a >> smaller address, or at the start. >> - when rhashtable_walk_start() is called, it records that 'p' is not >> 'safe', meaning that it cannot be dereferenced. The revalidation >> that was previously done here is moved to rhashtable_walk_next() >> - when rhashtable_walk_next() is called while p is not NULL and not >> safe, it walks the chain looking for the first object with an >> address greater than p and returns that. If there is none, it moves >> to the next hash chain. >>=20 >> Signed-off-by: NeilBrown >> --- >>=20 >> This is a resend of a patch that I sent back in July. I couldn't >> applied then because it assumed another rhashtable patch which hadn't >> landed yet - it now has. > > I thought we had agreed to drop this because nobody needs it > currently and it doesn't handle rhlist? Hi Herbert, I think it was agreed that I would not pursue features that were only of use to out-of-tree code, but I don't think that applies here. This is not a feature, this is a quality-of-implementation improvement. There are users in the kernel today which use rhashtable_walk_stop()/rhashtable_walk_start() to drop out of RCU protection for periods during the walk. Any such user might miss seeing an object that has been in the table for a while - sure that is less than optimal, and should be fixed if the cost is small. There are currently no rhlist users which use stop/start to drop out of RCU, so there is no clear value in fixing that case, or cost in not fixing it. Thanks, NeilBrown > > Cheers, > --=20 > Email: Herbert Xu > Home Page: http://gondor.apana.org.au/~herbert/ > PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAlwNnEkACgkQOeye3VZi gbmlcw/+ISsgWh2pLN5zi0zlPyCtmS9HBgLVORMI7x4A1ukG+XN7bwMKJYYFVlwM 5h5uJy74UREnkyosscX06yjQpc35Xd0vYUbHsdp3IhrMEmWDdjw/aZ1/kKN81XAy /aCfBgbNqckEiahfgBOrCakMmkkRYW3O80FKLccp2ROqTYB7k1kinWPi+WWH3VLo TUEfxvJ5F+/tpANsu6SjoARJyEC1arkb+ZVIGON7nmkU6F8z++CA8J7zwnhCUv82 1ronoV1YsKB5i8qRu//QRnDaJtMeMrAApKZcaqY5SihvuKqu61CE/+yPKoMtrhtX SNShhjsrcaOAUVcRtzJ4Ed05WKJJXIqa9N1Mc1EVoOdD/BdkWqhDgtmOt8lFXbFN HWf1g9B3czA5m33/+lU8tqfk3VZCFRu8/TXOXbM4CdZN1qSFbthHcvSZtEsBKyP4 +G0TXB/gMR0dmxeaZ6IUthWbjQRhVEHHVHtxfkIrAFdCCUVxbttQFwHvJKjf1otO amcTtxu65SYOb822I7UQTJbrTvYaf5TQLBBN5ro+hWQ5vTYXYKAHDlqIZqK+SAAa p2LBp0eRel2xF+iOZEdyoDyVEAVGP8o6+48ZGvGuEHN4fzk31WzDnIBtkhE4uNkg 3OSyIEiiDn5VvAkebnuooS6d4CuD3ENMSK0lzh0AjlpWjA9gqM4= =Z99R -----END PGP SIGNATURE----- --=-=-=--