linux-kernel - Re: [PATCH v8 6/7] apei/ghes: Use unrcu

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Date:   Fri, 14 Oct 2022 17:10:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Justin He <Justin.He@....com>, Borislav Petkov <bp@...en8.de>,
        Len Brown <lenb@...nel.org>, James Morse <James.Morse@....com>,
        Tony Luck <tony.luck@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Robert Richter <rric@...nel.org>,
        Robert Moore <robert.moore@...el.com>,
        Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Jan Luebbe <jlu@...gutronix.de>,
        Khuong Dinh <khuong@...amperecomputing.com>,
        Kani Toshi <toshi.kani@....com>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "devel@...ica.org" <devel@...ica.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Shuai Xue <xueshuai@...ux.alibaba.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

On Fri, Oct 14, 2022 at 04:31:37PM +0200, Ard Biesheuvel wrote:
> +       if (slot != -1) {
> +               /*
> +                * Use release semantics to ensure that ghes_estatus_cached()
> +                * running on another CPU will see the updated cache fields if
> +                * it can see the new value of the pointer.
> +                */
> +               victim = xchg_release(ghes_estatus_caches + slot,
> +                                     RCU_INITIALIZER(new_cache));
> +
> +               /*
> +                * At this point, victim may point to a cached item different
> +                * from the one based on which we selected the slot. Instead of
> +                * going to the loop again to pick another slot, let's just
> +                * drop the other item anyway: this may cause a false cache
> +                * miss later on, but that won't cause any problems.
> +                */
> +               if (victim) {
> +                       call_rcu(&rcu_dereference(victim)->rcu,
> +                                ghes_estatus_cache_rcu_free);
		}

I think you can use unrcu_pointer() here instead, there should not be a
data dependency since the ->rcu member itself should be otherwise unused
(and if it were, we wouldn't care about its previous content anyway).

But only Alpha cares about that distinction anyway, so *shrug*.

While I much like the xchg() variant; I still don't really fancy the
verbage the sparse nonsense makes us do.

		victim = xchg_release(&ghes_estatus_caches[slot], new_cache);
		if (victim)
			call_rcu(&victim->rcu, ghes_estatus_cache_rcu_free);

is much nicer code.

Over all; I'd simply ignore sparse (I often do).

> +       }
>         rcu_read_unlock();
>  }