Date: Sun, 24 Aug 2014 19:42:36 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: atomic_load_acq_int in sequential_heuristic Message-ID: <20140824164236.GX2737@kib.kiev.ua> In-Reply-To: <20140824162331.GW2737@kib.kiev.ua> References: <20140824115729.GC2045@dft-labs.eu> <20140824162331.GW2737@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
--oNcod2irK9ys8Qxe Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sun, Aug 24, 2014 at 07:23:31PM +0300, Konstantin Belousov wrote: > On Sun, Aug 24, 2014 at 01:57:29PM +0200, Mateusz Guzik wrote: > > Writer side is: > > fp->f_seqcount =3D (arg + bsize - 1) / bsize; > > do { > > new =3D old =3D fp->f_flag; > > new |=3D FRDAHEAD; > > } while (!atomic_cmpset_rel_int(&fp->f_flag, old, new)); > >=20 > > Reader side is: > > if (atomic_load_acq_int(&(fp->f_flag)) & FRDAHEAD) > > return (fp->f_seqcount << IO_SEQSHIFT); > >=20 > > We can easily get the following despite load_acq: > > CPU0 CPU1 > > load_acq fp->f_flag > > fp->f_seqcount =3D ... > > store_rel fp->f_flag > > read fp->f_seqcount > > =09 > > So the barrier does not seem to serve any purpose. > It does. >=20 > Consider initial situation, when the flag is not set yet. There, we > do not want to allow the reader to interpret automatically calculated > f_seqcount as the user-supplied constant. Without barriers, we might > read the flag as set, while user-provided value for f_seqcount is still > not visible to processor doing read. That said, I think now that there is a real bug. If we did not read the FRDAHEAD in sequential_heuristic(), we may override user-supplied value for f_seqcount. I do not see other solution than start to use locking. --oNcod2irK9ys8Qxe Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBAgAGBQJT+hX8AAoJEJDCuSvBvK1BgmAQAJNQ0m8/urD0kssB0dO81XiS GKYSvzkAX2Ca+kQ6uHZm7JEBsSL8rBgUU/pLa4lUymnQoSjutz54YcuBIvxBnRV2 MZVUy+HqoW4Ze2XP3YQVLAyJjXhH9bVme9dye+btyXOf/qR8WXemGId5vTeZa61T gtAm4jcTPA0tkFFsAUoCBX/+//0ixVZb/YUQGdEPbd1YpModt06Lrgwm9K5mBGNm RZVZ8j0HsjDH6cHxvw4Z1ENwka/1gYA1bIBA1soXUU1RgKZf9kFArds5Y5jo7QTP iDQCquHmPenA1EJ1DUAj6/3EiIErL+Velsyv2xkA00yQgYCUiMk9wOV5lW3c0kCq QKSg2OMnE8SwODDX9QgFNtS2X91Qjj2UayUS4M5Hy1WDHcgzDDhlWipO95bwji9u hl0e2eheENST2DWZ4R6bqE9nmFpcGs8zsG79+2qc5geYpqv5RljgDUp8+STDfhG3 9ddZhv0mnsmRX14gE1AqB9o25XwRQpr4up99S1ilo/3B8Id7TQv0iNvEl3OD7G3U IOB+U0uQ+CxoM70DOGK90fcMzsnflGF0wySUuDEZyaTTqCJL4KnYJYS3ai9plVzM cGtLtdHv08FjsZpA1fy/8p4ePiBui2G3e1mf6s4p98KZSrdJeItop0Dkc4Ykj4X9 jQevNfkEGrydNmFDicfD =7/yO -----END PGP SIGNATURE----- --oNcod2irK9ys8Qxe--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20140824164236.GX2737>