Re: [PATCH V5] riscv: errata: Add ERRATA_THEAD_WRITE_ONCE fixup

From: Drew Fustini <fustini@kernel.org>
To: Paul Walmsley <pjw@kernel.org>
Cc: Guo Ren <guoren@kernel.org>, Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>, Arnd Bergmann <arnd@arndb.de>,
	Chen Wang <unicorn_wang@outlook.com>,
	Inochi Amaoto <inochiama@gmail.com>,
	Leonardo Bras <leobras@redhat.com>,
	Raj Vishwanathan4 <rvishwanathan@mips.com>,
	Djordje Todorovic <djordje.todorovic@htecgroup.com>,
	Alexandre Ghiti <alexghiti@rivosinc.com>,
	Han Gao <gaohan@iscas.ac.cn>, Yao Zi <me@ziyao.cc>,
	Xiaoguang Xing <xiaoguang.xing@sophgo.com>,
	Aleksandar Rikalo <arikalo@gmail.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, sophgo@lists.linux.dev
Subject: Re: [PATCH V5] riscv: errata: Add ERRATA_THEAD_WRITE_ONCE fixup
Date: Tue, 2 Jun 2026 10:08:09 -0700	[thread overview]
Message-ID: <ah8N+UThw4JnOdDb@x1> (raw)
In-Reply-To: <20260421143154.1590156-1-guoren@kernel.org>

On Tue, Apr 21, 2026 at 10:31:40AM -0400, guoren@kernel.org wrote:
> From: "Guo Ren (Alibaba DAMO Academy)" <guoren@kernel.org>
> 
> The early version of XuanTie C910 core has a store merge buffer
> delay problem. The store merge buffer could improve the store queue
> performance by merging multi-store requests, but when there are not
> continued store requests, the prior single store request would be
> waiting in the store queue for a long time. That would cause
> significant problems for communication between multi-cores. This
> problem was found on sg2042 & th1520 platforms with the qspinlock
> lock torture test.
> 
> So appending a fence w.o could immediately flush the store merge
> buffer and let other cores see the write result.
> 
> This will apply the WRITE_ONCE errata to handle the non-standard
> behavior via appending a fence w.o instruction for WRITE_ONCE().
> 
> This problem is only observed on the sg2042 hardware platform by
> running the lock_torture test program for half an hour. The problem
> was not found in the user space application, because interrupt can
> break the livelock.
> 
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Reviewed-by: Leonardo Bras <leobras@redhat.com>
> Reviewed-by: Inochi Amaoto <inochiama@gmail.com>
> Tested-by: Han Gao <gaohan@iscas.ac.cn>
> Tested-by: Yao Zi <me@ziyao.cc>
> Cc: Chen Wang <unicorn_wang@outlook.com>
> Cc: Xiaoguang Xing <xiaoguang.xing@sophgo.com>
> Cc: Paul Walmsley <pjw@kernel.org>
> Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@kernel.org>
> ---
> Changelog
> 
> v5:
>  - Add Acked-by: Arnd Bergmann <arnd@arndb.de> for asm/generic
>  - Add Reviewed-by: Inochi Amaoto <inochiama@gmail.com>
>  - Rebase on v7.0

Hi Paul,

Han Gao (revy) let me know that there is now an errata entry for 'Writes
might stick in C910/C920v1 store buffer for too long' [1].

Does that address the concern you had about documenting the errata?

Thanks,
Drew

[1] https://github.com/revyos/xuantie-c900-bugs#writes-might-stick-in-c910c920v1-store-buffer-for-too-long