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From: "H. Peter Anvin" <hpa@zytor.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: paulmck@linux.vnet.ibm.com,
	Peter Hurley <peter@hurleysoftware.com>,
	One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
	Jakub Jelinek <jakub@redhat.com>,
	Mikael Pettersson <mikpelinux@gmail.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Richard Henderson <rth@twiddle.net>,
	Oleg Nesterov <oleg@redhat.com>,
	Miroslav Franc <mfranc@redhat.com>,
	Paul Mackerras <paulus@samba.org>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, Tony Luck <tony.luck@intel.com>,
	linux-ia64@vger.kernel.org
Subject: Re: bit fields && data tearing
Date: Mon, 08 Sep 2014 19:30:29 -0700	[thread overview]
Message-ID: <540E6645.6040209@zytor.com> (raw)
In-Reply-To: <1410215994.2027.86.camel@jarvis.lan>

On 09/08/2014 03:39 PM, James Bottomley wrote:
> 
> I don't understand what you mean by "pass each other".  Atomicity
> guarantees are not ordering guarantees in a SMP environment.  The
> guarantee is that if you follow the rules when two CPUs update the same
> natural width aligned object simultaneously using the same primitive,
> the result is either one or the other of their updates.  Which one wins
> (the ordering) isn't defined.
> 

I'm trying to figure out why it would possibly make a difference in any
kind of sane system if gcc fuses accesses.

Assuming bigendian for the moment, I would expect that if CPU 1 does a
write of 0x01020304 to address 0 and CPU 2 does a write of 0x0506 to
address 2, that the end result would be either 0x01020304 or 0x01020506.
 Similarly, I would expect that if these operations are both done on the
same CPU in that order, that the result would unambiguously be 0x01020506.

I would strongly suspect an architecture which does not provide those
guarantees is an outlier.

	-hpa



      reply	other threads:[~2014-09-09  2:35 UTC|newest]

Thread overview: 103+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-12 18:13 bit fields && data tearing Oleg Nesterov
2014-07-12 20:51 ` Oleg Nesterov
2014-07-12 23:34   ` Benjamin Herrenschmidt
2014-07-13 12:29     ` Oleg Nesterov
2014-07-13 13:15     ` Peter Hurley
2014-07-13 22:25       ` Benjamin Herrenschmidt
2014-07-15 13:54         ` Peter Hurley
2014-07-15 15:02           ` Richard Henderson
2014-09-03 22:51 ` Peter Hurley
2014-09-03 23:11   ` Benjamin Herrenschmidt
2014-09-04  8:43     ` David Laight
2014-09-04  9:52       ` Benjamin Herrenschmidt
2014-09-04 22:14         ` H. Peter Anvin
2014-09-05  0:59           ` Peter Hurley
2014-09-05  2:08             ` H. Peter Anvin
2014-09-05 15:31               ` Peter Hurley
2014-09-05 15:41                 ` H. Peter Anvin
2014-09-08 17:52                   ` One Thousand Gnomes
2014-09-08 17:59                     ` H. Peter Anvin
2014-09-08 19:17                       ` One Thousand Gnomes
2014-09-09 11:18                         ` Peter Hurley
2014-09-08 22:47                       ` Peter Hurley
2014-09-09  1:59                         ` Paul E. McKenney
2014-09-09 11:14                         ` Peter Hurley
2014-09-11 10:04                         ` One Thousand Gnomes
2014-09-11 16:16                           ` Paul E. McKenney
2014-09-11 20:01                           ` Peter Hurley
2014-09-14 23:24                             ` One Thousand Gnomes
2014-09-22 19:51                               ` Paul E. McKenney
2014-09-23 18:19                               ` Peter Hurley
2014-09-23 18:39                                 ` One Thousand Gnomes
2014-09-08 18:13                     ` James Bottomley
2014-09-10 20:18                     ` H. Peter Anvin
2014-09-10 21:10                       ` Rob Landley
2014-09-05  2:08             ` H. Peter Anvin
2014-09-05  8:16               ` Michael Cree
2014-09-05 18:09                 ` Paul E. McKenney
2014-09-05 18:31                   ` Paul E. McKenney
2014-09-05 19:52                     ` Peter Zijlstra
2014-09-05 20:01                       ` Peter Hurley
2014-09-05 20:12                         ` Peter Zijlstra
2014-09-05 20:15                           ` H. Peter Anvin
2014-09-05 20:19                         ` Paul E. McKenney
2014-09-05 18:50                   ` Peter Hurley
2014-09-05 19:05                     ` Paul E. McKenney
2014-09-05 19:24                       ` Peter Hurley
2014-09-05 20:09                         ` Paul E. McKenney
2014-09-05 19:38                       ` Marc Gauthier
2014-09-05 20:14                         ` Peter Hurley
2014-09-05 20:34                           ` H. Peter Anvin
2014-09-05 20:42                             ` Michael Cree
2014-09-05 20:43                             ` Paul E. McKenney
2014-09-05 20:48                               ` Thomas Gleixner
2014-09-05 21:05                                 ` Paul E. McKenney
2014-09-05 20:39                           ` Michael Cree
2014-09-05 21:12                             ` Peter Hurley
2014-09-05 21:27                               ` Michael Cree
2014-09-05 20:42                           ` Paul E. McKenney
2014-09-04  8:57     ` Mikael Pettersson
2014-09-04  9:09       ` Jakub Jelinek
2014-09-04 12:24         ` Peter Hurley
2014-09-04 12:29           ` Jakub Jelinek
2014-09-04 16:50           ` One Thousand Gnomes
2014-09-04 19:42             ` Peter Hurley
2014-09-04 22:16               ` H. Peter Anvin
2014-09-05  0:17                 ` Paul E. McKenney
2014-09-05  1:57                   ` Peter Hurley
2014-09-05  2:11                   ` James Bottomley
2014-09-05  2:47                     ` Peter Hurley
2014-09-05  4:06                       ` Paul E. McKenney
2014-09-05  8:30                         ` David Laight
2014-09-05 12:31                           ` Peter Hurley
2014-09-05 12:37                             ` David Laight
2014-09-05 16:17                               ` Peter Hurley
2014-09-25 16:12                                 ` Pavel Machek
2014-09-07  5:07                         ` James Bottomley
2014-09-07 16:21                           ` Paul E. McKenney
2014-09-07 19:04                             ` James Bottomley
2014-09-07 20:41                               ` Peter Hurley
2014-09-08  5:50                                 ` James Bottomley
2014-09-08 20:45                                   ` Chris Metcalf
2014-09-08 22:43                                     ` James Bottomley
2014-09-09  2:27                                       ` H. Peter Anvin
2014-09-09  8:11                                         ` Arnd Bergmann
2014-09-08 23:30                                   ` Peter Hurley
2014-09-09  2:56                                     ` James Bottomley
2014-09-09  3:20                                       ` H. Peter Anvin
2014-09-09  4:30                                       ` H. Peter Anvin
2014-09-09 10:40                                       ` Peter Hurley
2014-09-10 21:48                                         ` James Bottomley
2014-09-10 23:50                                           ` Peter Hurley
2014-09-11 10:23                                           ` Will Deacon
2014-09-07 23:00                               ` Paul E. McKenney
2014-09-07 23:17                                 ` H. Peter Anvin
2014-09-07 23:36                                   ` Paul E. McKenney
2014-09-07 23:39                                     ` H. Peter Anvin
2014-09-08  5:56                                       ` James Bottomley
2014-09-08 18:12                                         ` H. Peter Anvin
2014-09-08 19:09                                           ` James Bottomley
2014-09-08 19:12                                             ` H. Peter Anvin
2014-09-08 19:12                                             ` H. Peter Anvin
2014-09-08 22:39                                               ` James Bottomley
2014-09-09  2:30                                                 ` H. Peter Anvin [this message]

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