From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Michael Cree <mcree@orcon.net.nz>,
"H. Peter Anvin" <hpa@zytor.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
David Laight <David.Laight@ACULAB.COM>,
Jakub Jelinek <jakub@redhat.com>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
Tony Luck <tony.luck@intel.com>,
"linux-ia64@vger.kernel.org" <linux-ia64@vger.kernel.org>,
Oleg Nesterov <oleg@redhat.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Paul Mackerras <paulus@samba.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
Miroslav Franc <mfranc@redhat.com>,
Richard Henderson <rth@twiddle.net>,
linux-alpha@vger.kernel.org
Subject: Re: bit fields && data tearing
Date: Fri, 5 Sep 2014 12:05:06 -0700 [thread overview]
Message-ID: <20140905190506.GV5001@linux.vnet.ibm.com> (raw)
In-Reply-To: <540A05F7.1070202@hurleysoftware.com>
On Fri, Sep 05, 2014 at 02:50:31PM -0400, Peter Hurley wrote:
> On 09/05/2014 02:09 PM, Paul E. McKenney wrote:
> > On Fri, Sep 05, 2014 at 08:16:48PM +1200, Michael Cree wrote:
> >> On Thu, Sep 04, 2014 at 07:08:48PM -0700, H. Peter Anvin wrote:
> >>> On 09/04/2014 05:59 PM, Peter Hurley wrote:
> >>>> I have no idea how prevalent the ev56 is compared to the ev5.
> >>>> Still we're talking about a chip that came out in 1996.
> >>>
> >>> Ah yes, I stand corrected. According to Wikipedia, the affected CPUs
> >>> were all the 2106x CPUs (EV4, EV45, LCA4, LCA45) plus the 21164 with no
> >>> suffix (EV5). However, we're still talking about museum pieces here.
> >>
> >> Yes, that is correct, EV56 is the first Alpha CPU to have the byte-word
> >> extension (BWX) CPU instructions.
> >>
> >> It would not worry me if the kernel decided to assume atomic aligned
> >> scalar accesses for all arches, thus terminating support for Alphas
> >> without BWX.
> >>
> >> The X server, ever since the libpciaccess change, does not work on
> >> Alphas without BWX.
> >>
> >> Debian Alpha (pretty much up to date at Debian-Ports) is still compiled
> >> for all Alphas, i.e., without BWX. The last attempt to start compiling
> >> Debian Alpha with BWX, about three years ago when Alpha was kicked out
> >> to Debian-Ports resulted in a couple or so complaints so got nowhere.
> >> It's frustrating supporting the lowest common demoninator as many of
> >> the bugs specific to Alpha can be resolved by recompiling with the BWX.
> >> The kernel no longer supporting Alphas without BWX might just be the
> >> incentive we need to switch Debian Alpha to compiling with BWX.
> >
> > Very good, then I update my patch as follows. Thoughts?
> >
> > Thanx, Paul
>
> Minor [optional] edits.
>
> Thanks,
> Peter Hurley
>
> > ------------------------------------------------------------------------
> >
> > documentation: Record limitations of bitfields and small variables
> >
> > This commit documents the fact that it is not safe to use bitfields as
> > shared variables in synchronization algorithms. It also documents that
> > CPUs must provide one-byte and two-byte load and store instructions
> ^
> atomic
Here you meant non-atomic? My guess is that you are referring to the
fact that you could emulate a one-byte store on pre-EV56 Alpha CPUs
using the ll and sc atomic-read-modify-write instructions, correct?
> > in order to be supported by the Linux kernel. (Michael Cree
> > has agreed to the resulting non-support of pre-EV56 Alpha CPUs:
> > https://lkml.org/lkml/2014/9/5/143.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 87be0a8a78de..455df6b298f7 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -269,6 +269,30 @@ And there are a number of things that _must_ or _must_not_ be assumed:
> > STORE *(A + 4) = Y; STORE *A = X;
> > STORE {*A, *(A + 4) } = {X, Y};
> >
> > +And there are anti-guarantees:
> > +
> > + (*) These guarantees do not apply to bitfields, because compilers often
> > + generate code to modify these using non-atomic read-modify-write
> > + sequences. Do not attempt to use bitfields to synchronize parallel
> > + algorithms.
> > +
> > + (*) Even in cases where bitfields are protected by locks, all fields
> > + in a given bitfield must be protected by one lock. If two fields
> > + in a given bitfield are protected by different locks, the compiler's
> > + non-atomic read-modify-write sequences can cause an update to one
> > + field to corrupt the value of an adjacent field.
> > +
> > + (*) These guarantees apply only to properly aligned and sized scalar
> > + variables. "Properly sized" currently means variables that are the
> > + same size as "char", "short", "int" and "long". "Properly aligned"
> > + means the natural alignment, thus no constraints for "char",
> > + two-byte alignment for "short", four-byte alignment for "int",
> > + and either four-byte or eight-byte alignment for "long", on 32-bit
> > + and 64-bit systems, respectively. Note that this means that the
> > + Linux kernel does not support pre-EV56 Alpha CPUs, because these
> > + older CPUs do not provide one-byte and two-byte loads and stores.
> ^
> non-atomic
I took this, thank you!
Thanx, Paul
> > + Alpha EV56 and later Alpha CPUs are still supported.
> > +
> >
> > =========================
> > WHAT ARE MEMORY BARRIERS?
> >
>
next prev parent reply other threads:[~2014-09-05 19:05 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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20140905190506.GV5001@linux.vnet.ibm.com \
--to=paulmck@linux.vnet.ibm.com \
--cc=David.Laight@ACULAB.COM \
--cc=benh@kernel.crashing.org \
--cc=hpa@zytor.com \
--cc=jakub@redhat.com \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-ia64@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=mcree@orcon.net.nz \
--cc=mfranc@redhat.com \
--cc=oleg@redhat.com \
--cc=paulus@samba.org \
--cc=peter@hurleysoftware.com \
--cc=rth@twiddle.net \
--cc=tony.luck@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).