Received-SPF: pass (google.com: domain of linux-kernel+bounces-177913-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-Id: <202405131652.44DGqMjs007653@valdese.nms-ulrich-teichert.org>
Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10
To: akiyks@gmail.com (Akira Yokosawa)
Date: Mon, 13 May 2024 18:52:22 +0200 (CEST)
Cc: paulmck@kernel.org, arnd@arndb.de, glaubitz@physik.fu-berlin.de,
        ink@jurassic.park.msu.ru, linux-alpha@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        mattst88@gmail.com, richard.henderson@linaro.org,
        torvalds@linux-foundation.org, viro@zeniv.linux.org.uk,
        krypton@ulrich-teichert.org (Ulrich Teichert),
        akiyks@gmail.com (Akira Yokosawa)
In-Reply-To: <99765904-3f35-4c78-998e-b444a6ab90e4@gmail.com>
From: Ulrich Teichert <krypton@ulrich-teichert.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Hi,

> On Sun, 12 May 2024 07:44:25 -0700, Paul E. McKenney wrote:
> > On Sun, May 12, 2024 at 08:02:59AM +0200, John Paul Adrian Glaubitz wrote:
> >> On Sat, 2024-05-11 at 18:26 -0700, Paul E. McKenney wrote:
> >> > And that breaks things because it can clobber concurrent stores to
> >> > other bytes in that enclosing machine word.
> >> 
> >> But pre-EV56 Alpha has always been like this. What makes it broken
> >> all of a sudden?
> > 
> > I doubt if it was sudden.   Putting concurrently (but rarely) accessed
> > small-value quantities into single bytes is a very natural thing to do,
> > and I bet that there are quite a few places in the kernel where exactly
> > this happens.  I happen to know of a specific instance that went into
> > mainline about two years ago.
> > 
> > So why didn't the people running current mainline on pre-EV56 Alpha
> > systems notice?  One possibility is that they are upgrading their
> > kernels only occasionally.  Another possibility is that they are seeing
> > the failures, but are not tracing the obtuse failure modes back to the
> > change(s) in question.  Yet another possibility is that the resulting
> > failures are very low probability, with mean times to failure that are
> > so long that you won't notice anything on a single system.
> 
> Another possibility is that the Jensen system was booted into uni processer
> mode.  Looking at the early boot log [1] provided by Ulrich (+CCed) back in
> Sept. 2021, I see the following by running "grep -i cpu":
> 
> >> > [1] https://marc.info/?l=linux-alpha&m=163265555616841&w=2
> 
> [    0.000000] Memory: 90256K/131072K available (8897K kernel code, 9499K rwdata, \
> 2704K rodata, 312K init, 437K bss, 40816K reserved, 0K cma-reserved) [    0.000000] \
> random: get_random_u64 called from __kmem_cache_create+0x54/0x600 with crng_init=0 [  \
> 0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1 [    0.000000]
>                                                      ^^^^^^
> 
> Without any concurrent atomic updates, the "broken" atomic accesses won't
> matter, I guess.

I've probably disabled SMP in my test kernel, the jensen is a single CPU
system. I never had the pleasure of owning an AlphaServer 2000 or 2100,
which (according to https://en.wikipedia.org/wiki/AlphaServer and
https://en.wikipedia.org/wiki/AlphaStation) are the only systems
with EV4/EV45/EV5 multi-CPU setups (apart from the Cray T3{DE}), so
the possibility of ever seeing an error concerning atomic concurrent
updates is quite low.

Anybody out there with an AlphaServer 2000/2100 willing to try ?-)

CU,
Uli
-- 
Dipl. Inf. Ulrich Teichert|e-mail: Ulrich.Teichert@gmx.de | Listening to:
Stormweg 24               |The Hives: Two Kinds Of Trouble, The Chats: 6L GTR,
24539 Neumuenster, Germany|La Fraction: Les Démons, Nightwatchers: On a Mission