Re: [GIT PULL] alpha: cleanups and build fixes for 6.10

From: Akira Yokosawa
Date: Sun May 12 2024 - 23:50:21 EST


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.

Thanks, Akira