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Date: Mon, 13 May 2024 08:28:32 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Paul E. McKenney" <paulmck@kernel.org>,
 "Akira Yokosawa" <akiyks@gmail.com>
Cc: "John Paul Adrian Glaubitz" <glaubitz@physik.fu-berlin.de>,
 "Ivan Kokshaysky" <ink@jurassic.park.msu.ru>, linux-alpha@vger.kernel.org,
 Linux-Arch <linux-arch@vger.kernel.org>, linux-kernel@vger.kernel.org,
 "Matt Turner" <mattst88@gmail.com>,
 "Richard Henderson" <richard.henderson@linaro.org>,
 "Linus Torvalds" <torvalds@linux-foundation.org>,
 "Alexander Viro" <viro@zeniv.linux.org.uk>,
 "Ulrich Teichert" <krypton@ulrich-teichert.org>
Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10
Content-Type: text/plain

On Mon, May 13, 2024, at 06:03, Paul E. McKenney wrote:
> On Mon, May 13, 2024 at 12:50:07PM +0900, Akira Yokosawa wrote:
>> 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:
>> > 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.
>
> True enough!

On the other hand, you would get the same broken behavior on
any SMP machine running a kernel that has support for EV5 or
earlier enabled in a multiplatform kernel. It doesn't really
matter if it's running on hardware that supports BWX or not
as long as the compiler doesn't generate those instructions.

If I understand it correctly, simply running rcutorture on
a large alpha machine with a 'defconfig' kernel from the
past two years should trigger some bugs even if you don't
run into them that frequently on light usage, right?

     Arnd