Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg

From: Peter Zijlstra <peterz@infradead.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Justin He <Justin.He@arm.com>, Borislav Petkov <bp@alien8.de>,
	Len Brown <lenb@kernel.org>, James Morse <James.Morse@arm.com>,
	Tony Luck <tony.luck@intel.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Robert Richter <rric@kernel.org>,
	Robert Moore <robert.moore@intel.com>,
	Qiuxu Zhuo <qiuxu.zhuo@intel.com>,
	Yazen Ghannam <yazen.ghannam@amd.com>,
	Jan Luebbe <jlu@pengutronix.de>,
	Khuong Dinh <khuong@os.amperecomputing.com>,
	Kani Toshi <toshi.kani@hpe.com>,
	"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>,
	"devel@acpica.org" <devel@acpica.org>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Shuai Xue <xueshuai@linux.alibaba.com>,
	Jarkko Sakkinen <jarkko@kernel.org>,
	"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
	kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v8 6/7] apei/ghes: Use unrcu_pointer for cmpxchg
Date: Fri, 14 Oct 2022 17:10:57 +0200	[thread overview]
Message-ID: <Y0l8AeQCrMLYW6g3@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAMj1kXHrP_P79ObKPFFgpN-X7gN+zaN1vKbsQZTJGvm=Uoav3g@mail.gmail.com>

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();
>  }