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

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Message-ID: <Y0m7KpdSAi6wIgfb@zn.tnic>
Date:   Fri, 14 Oct 2022 21:40:26 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Huang Ying <ying.huang@...el.com>,
        Justin He <Justin.He@....com>, 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>,
        nd <nd@....com>, 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 11:40:39AM +0200, Ard Biesheuvel wrote:
> The cache struct pointer should not be published until after the
> struct itself is fully populated. So on the producer side, some kind
> of hardware barrier is definitely needed, or the struct may appear
> half-baked to other cores that can read the updated pointer.

Ah, right you are, ghes_estatus_cached() is called in all kinds of
contexts and by other cores, sure.

> OTOH the code seems to be working fine as is, so why modify it at all?
> (apart from the purely cosmetic changes)

peterz questioned that smp_wmb() there and then we started digging. And
frankly, even if removing that barrier won't make any difference, I'd
still prefer it gone and have the code simpler and cleaner.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette