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Date: Sun, 12 May 2024 21:03:18 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Akira Yokosawa <akiyks@gmail.com>
Cc: 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,
	Ulrich Teichert <krypton@ulrich-teichert.org>
Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10
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 <99765904-3f35-4c78-998e-b444a6ab90e4@gmail.com>
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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:
> >> 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.

True enough!

							Thanx, Paul