From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
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>,
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: Thu, 30 May 2024 17:10:58 -0700 [thread overview]
Message-ID: <CAHk-=whiH6g+T7+YWSYgAhJ9HsJ2bUUDJfLLo_Yhbi8CVgkHDg@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2405302115130.23854@angie.orcam.me.uk>
On Thu, 30 May 2024 at 15:57, Maciej W. Rozycki <macro@orcam.me.uk> wrote:
>
> On Wed, 29 May 2024, Linus Torvalds wrote:
> >
> > The 21064 actually did atomicity with an external pin on the bus, the
> > same way people used to do before caches even existed.
>
> Umm, 8086's LOCK#, anyone?
Well, yes and no.
So yes, exactly like 8086 did before having caches.
But no, not like the alpha contemporary PPro that did have caches. The
PPro already did locked cycles in the caches.
Yes, the PPro still did have an external lock pin (and in fact current
much more modern x86 CPUs do too), but it's only used for locked IO
accesses or possibly cacheline crossing accesses.
So x86 has supported atomic accesses on IO - and it is very very slow,
to this day. So slow, and problematic, in fact, that Intel is only now
trying to remove it (look up "split lock"
But the 21064 explicitly did not support locking on IO - and unaligned
LL/SC accesses obviously also did not work.
So I really feel the 21064 was broken.
It's probably related to the whole cache coherency being designed to
be external to the built-in caches - or even the Bcache. The caches
basically are write-through, and the weak memory ordering was designed
for allowing this horrible model.
> > In fact, it's worse than "not thread safe". It's not even safe on UP
> > with interrupts, or even signals in user space.
>
> Ouch, I find it a surprising oversight.
The sad part is that it doesn't seem to have been an oversight. It
really was broken-as-designed.
Basically, the CPU was designed for single-threaded Spec benchmarks
and absolutely nothing else. Classic RISC where you recompile to fix
problems like the atomicity thing - "just use a 32-bit sig_atomic_t
and you're fine")
The original alpha architecture handbook makes a big deal of how
clever the lack of byte and word operations is. I also remember
reading an article by Dick Sites - one of the main designers - talking
a lot about how the lack of byte operations is great, and encourages
vectorizing byte accesses and doing string operations in whole words.
Linus
next prev parent reply other threads:[~2024-05-31 0:11 UTC|newest]
Thread overview: 55+ 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
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 [this message]
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
2024-05-31 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
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