From: "Maciej W. Rozycki" <macro@orcam.me.uk>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
Arnd Bergmann <arnd@kernel.org>,
linux-alpha@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
Richard Henderson <richard.henderson@linaro.org>,
Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
Matt Turner <mattst88@gmail.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Marc Zyngier <maz@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org, Michael Cree <mcree@orcon.net.nz>,
Frank Scheiner <frank.scheiner@web.de>
Subject: Re: [PATCH 00/14] alpha: cleanups for 6.10
Date: Fri, 31 May 2024 04:56:28 +0100 (BST) [thread overview]
Message-ID: <alpine.DEB.2.21.2405310432190.23854@angie.orcam.me.uk> (raw)
In-Reply-To: <5567ab2e-80af-4c5f-bebb-d979e8a34f49@paulmck-laptop>
On Wed, 29 May 2024, Paul E. McKenney wrote:
> > Mind that the read-modify-write sequence that software does for sub-word
> > write accesses with original Alpha hardware is precisely what hardware
> > would have to do anyway and support for that was deliberately omitted by
> > the architecture designers from the ISA to give it performance advantages
> > quoted in the architecture manual. The only difference here is that with
> > hardware read-modify-write operations atomicity for sub-word accesses is
> > guaranteed by the ISA, however for software read-modify-write it has to be
> > explictly coded using the usual load-locked/store-conditional sequence in
> > a loop. I don't think it's a big deal really, it should be trivial to do
> > in the relevant accessors, along with the memory barriers that are needed
> > anyway for EV56+ and possibly other ports such as the MIPS one.
>
> There shouldn't be any memory barriers required, and don't EV56+ have
> single-byte loads and stores?
I should have commented on this in my original reply.
You're the RCU expert so you know the answer. I don't. If it's OK for
successive writes to get reordered, or readers to see a stale value, then
you don't need memory barriers. Otherwise you do. Whether byte accesses
are available or not does not matter, the CPU *will* do reordering if it's
allowed to (or more specifically, it won't do anything to prevent it from
happening, especially in SMP configurations; I can't remember offhand if
there are cases with UP). Also adjacent byte writes may be merged, but I
suppose it does not matter, or does it?
NB MIPS has similar architectural arrangements (and a bunch of barriers
defined in the ISA), it's just most implementations are actually strongly
ordered, so most people can't see the effects of this. With MIPS I know
for sure there are cases of UP reordering, but they only really matter for
MMIO use cases and not regular memory.
Maciej
next prev parent reply other threads:[~2024-05-31 3:56 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-03 8:11 [PATCH 00/14] alpha: cleanups for 6.10 Arnd Bergmann
2024-05-03 8:11 ` [PATCH 01/14] alpha: sort scr_mem{cpy,move}w() out Arnd Bergmann
2024-05-03 8:11 ` [PATCH 02/14] alpha: fix modversions for strcpy() et.al Arnd Bergmann
2024-05-03 8:11 ` [PATCH 03/14] alpha: add clone3() support Arnd Bergmann
2024-05-03 8:11 ` [PATCH 04/14] alpha: don't make functions public without a reason Arnd Bergmann
2024-05-03 8:11 ` [PATCH 05/14] alpha: sys_sio: fix misspelled ifdefs Arnd Bergmann
2024-05-03 8:11 ` [PATCH 06/14] alpha: missing includes Arnd Bergmann
2024-05-03 8:11 ` [PATCH 07/14] alpha: core_lca: take the unused functions out Arnd Bergmann
2024-05-03 8:11 ` [PATCH 08/14] alpha: jensen, t2 - make __EXTERN_INLINE same as for the rest Arnd Bergmann
2024-05-03 8:11 ` [PATCH 09/14] alpha: trim the unused stuff from asm-offsets.c Arnd Bergmann
2024-05-03 8:11 ` [PATCH 10/14] alpha: remove DECpc AXP150 (Jensen) support Arnd Bergmann
2024-05-03 16:07 ` Linus Torvalds
2024-05-03 17:00 ` Al Viro
2024-05-03 20:07 ` Arnd Bergmann
2024-05-03 8:11 ` [PATCH 11/14] alpha: sable: remove early machine support Arnd Bergmann
2024-05-03 8:11 ` [PATCH 12/14] alpha: remove LCA and APECS based machines Arnd Bergmann
2024-05-03 8:11 ` [PATCH 13/14] alpha: cabriolet: remove EV5 CPU support Arnd Bergmann
2024-05-03 8:11 ` [PATCH 14/14] alpha: drop pre-EV56 support Arnd Bergmann
2024-05-04 15:00 ` Richard Henderson
2024-05-06 10:06 ` Arnd Bergmann
2024-06-03 6:02 ` Jiri Slaby
2024-06-04 13:58 ` Greg KH
2024-05-03 16:06 ` [PATCH 00/14] alpha: cleanups for 6.10 Matt Turner
2024-05-03 20:15 ` Arnd Bergmann
2024-05-06 9:16 ` Michael Cree
2024-05-06 10:11 ` Arnd Bergmann
2024-05-03 16:53 ` John Paul Adrian Glaubitz
2024-05-03 17:19 ` Paul E. McKenney
2024-05-27 23:49 ` Maciej W. Rozycki
2024-05-28 14:43 ` Paul E. McKenney
2024-05-29 18:50 ` Maciej W. Rozycki
2024-05-29 22:09 ` Paul E. McKenney
2024-05-30 22:59 ` Maciej W. Rozycki
2024-05-31 3:56 ` Maciej W. Rozycki [this message]
2024-05-31 19:33 ` Paul E. McKenney
2024-06-03 16:22 ` Maciej W. Rozycki
2024-06-03 17:08 ` Paul E. McKenney
2024-07-01 23:50 ` Maciej W. Rozycki
2024-05-30 1:08 ` Linus Torvalds
2024-05-30 22:57 ` Maciej W. Rozycki
2024-05-31 0:10 ` Linus Torvalds
2024-06-03 11:09 ` Maciej W. Rozycki
2024-06-03 11:36 ` John Paul Adrian Glaubitz
2024-06-03 16:57 ` Linus Torvalds
2024-07-01 23:48 ` Maciej W. Rozycki
2024-11-19 17:54 ` Maciej W. Rozycki
2024-05-31 15:48 ` Arnd Bergmann
2024-05-31 16:32 ` Linus Torvalds
2024-05-31 16:54 ` Arnd Bergmann
2024-06-01 13:51 ` David Laight
2024-07-01 23:48 ` Maciej W. Rozycki
2024-07-02 1:13 ` Linus Torvalds
2024-07-03 0:12 ` Maciej W. Rozycki
2024-07-03 0:50 ` Linus Torvalds
2024-07-04 22:21 ` Maciej W. Rozycki
2024-06-03 11:33 ` Maciej W. Rozycki
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=alpine.DEB.2.21.2405310432190.23854@angie.orcam.me.uk \
--to=macro@orcam.me.uk \
--cc=arnd@arndb.de \
--cc=arnd@kernel.org \
--cc=frank.scheiner@web.de \
--cc=glaubitz@physik.fu-berlin.de \
--cc=ink@jurassic.park.msu.ru \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mattst88@gmail.com \
--cc=maz@kernel.org \
--cc=mcree@orcon.net.nz \
--cc=paulmck@kernel.org \
--cc=richard.henderson@linaro.org \
--cc=torvalds@linux-foundation.org \
--cc=viro@zeniv.linux.org.uk \
/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 read-only IMAP folder(s) and NNTP newsgroup(s).