linux-kernel - Re: [RFC][PATCH 11/12] slub: Replace cmpxchg

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Message-ID: <CAHk-=wj9nK825MyHXu7zkegc7Va+ZxcperdOtRMZBCLHqGrr=g@mail.gmail.com>
Date:   Tue, 3 Jan 2023 11:08:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, corbet@....net,
        will@...nel.org, boqun.feng@...il.com, mark.rutland@....com,
        catalin.marinas@....com, dennis@...nel.org, tj@...nel.org,
        cl@...ux.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        joro@...tes.org, suravee.suthikulpanit@....com,
        robin.murphy@....com, dwmw2@...radead.org,
        baolu.lu@...ux.intel.com, Arnd Bergmann <arnd@...db.de>,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        Andrew Morton <akpm@...ux-foundation.org>, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux.dev,
        linux-arch@...r.kernel.org
Subject: Re: [RFC][PATCH 11/12] slub: Replace cmpxchg_double()

On Tue, Jan 3, 2023 at 9:17 AM Heiko Carstens <hca@...ux.ibm.com> wrote:
>
> On Mon, Dec 19, 2022 at 04:35:36PM +0100, Peter Zijlstra wrote:
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  include/linux/slub_def.h |   12 ++-
> >  mm/slab.h                |   41 +++++++++++--
> >  mm/slub.c                |  146 ++++++++++++++++++++++++++++-------------------
> >  3 files changed, 135 insertions(+), 64 deletions(-)
>
> Does this actually work? Just wondering since I end up with an instant
> list corruption on s390. Might be endianness related, but I can't see
> anything obvious at a first glance.

I don't see anything that looks related to endianness, because while
there is that 128-bit union member, it's always either used in full,
or it's accessed as other union members.

But I *do* note that this patch seems to be the only one that depends
on the new this_cpu_cmpxchg() updates to make it just automatically do
the right thing for a 128-bit value. And I have to admit that all
those games with __pcpu_cast_128() make no sense to me. Why isn't it
just using "u128" everywhere without any odd _Generic() games?

I could also easily see that if the asm constraints are wrong (like
the "cast pointer to (unsigned long *) instead of keeping it pointing
to a 128-bit type" thing discussed earlier), then code like this:

+       freelist_aba_t old = { .freelist = freelist_old, .counter = tid };
+       freelist_aba_t new = { .freelist = freelist_new, .counter =
next_tid(tid) };
+
+       return this_cpu_cmpxchg(s->cpu_slab->freelist_tid.full,
+                               old.full, new.full) == old.full;

would easily make the compiler go "the second word of 'old' is never
used by the asm, so I won't initialize it".

But yeah, that patch is hard to read, so hard to say. Does everything
leading up to it work fine?

                 Linus