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Message-ID: <Y0l8AeQCrMLYW6g3@hirez.programming.kicks-ass.net>
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();
> }
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