Re: [PATCH 1/2] riscv: unaligned: stop using kthread for check_vector_unaligned_access()

From: Paul Walmsley <pjw@kernel.org>
To: Nam Cao <namcao@linutronix.de>
Cc: Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	 Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	 Andrew Jones <andrew.jones@oss.qualcomm.com>,
	 Jingwei Wang <wangjingwei@iscas.ac.cn>,
	 Anirudh Srinivasan <asrinivasan@oss.tenstorrent.com>,
	 linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	 stable@vger.kernel.org
Subject: Re: [PATCH 1/2] riscv: unaligned: stop using kthread for check_vector_unaligned_access()
Date: Wed, 29 Jul 2026 13:31:48 -0600 (MDT)	[thread overview]
Message-ID: <dd5ff254-fc7c-ddd2-751a-50e6971b10af@kernel.org> (raw)
In-Reply-To: <1c378963f27c5960e8a57c50b8b444d30954cb54.1781666867.git.namcao@linutronix.de>

Hi,

On Wed, 17 Jun 2026, Nam Cao wrote:

> A kthread is used to run check_vector_unaligned_access() to optimize boot
> time, allowing the kernel to continue booting without waiting for the
> unaligned vector speed probe to finish.
> 
> However, this asynchronous approach introduces several complications.
> First, the kthread may not complete before a user reads vDSO data,
> resulting in incorrect values. This was previously addressed by
> commit 5d15d2ad36b0 ("riscv: hwprobe: Fix stale vDSO data for
> late-initialized keys at boot"), which added complex synchronization
> between the kthread and vDSO reads.
> 
> Second, it was discovered that the kthread may not finish before
> vec_check_unaligned_access_speed_all_cpus() (marked with __init) is freed,
> triggering a page fault.
> 
> These issues raise the question of whether the kthread is worth the added
> complexity. A past boot time regression report was actually unrelated to
> synchronous probing; it was caused by the probe running serially. Since
> switching to a parallel probe, no further complaints have been made.
> Furthermore, the unaligned scalar access speed probe takes the same amount
> of time, runs synchronously, and has caused no issues.
> 
> Testing shows no noticeable boot time slowdown when running the vector
> probe synchronously (0.464474s with kthread vs. 0.457991s without).
> 
> Remove the kthread usage and run the probe synchronously. This simplifies
> the boot flow and allows for the revert of commit 5d15d2ad36b0 ("riscv:
> hwprobe: Fix stale vDSO data for late-initialized keys at boot")
>
> Reported-by: Anirudh Srinivasan <asrinivasan@oss.tenstorrent.com>
> Closes: https://lore.kernel.org/linux-riscv/20260612-vec_unaligned_drop_init-v1-1-df969210ae34@oss.tenstorrent.com/
> Fixes: a00e022be531 ("riscv: Annotate unaligned access init functions")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Nam Cao <namcao@linutronix.de>

Thanks, will queue this for v7.3, but I think the Fixes: tag should point 
to e7c9d66e313b ("RISC-V: Report vector unaligned access speed hwprobe"), 
since the problems that you mention are due to the use of kthreads.  
So I'll plan to update this accordingly when it's queued.

- Paul

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