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

[Nam Cao <namcao@linutronix.de>]

Nam Cao <namcao@linutronix.de> writes:
> 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.

Another point I forgot to include. We start the kthread to run
asynchronously, but the probe is executed on all CPUs including the boot
CPU. Therefore if the kthread is executed before boot is completed,
asynchronous probe will actually slow down boot time due to the overhead
with kthread. If the kthread is executed after boot is completed, we run
into the two race conditions mentioned above.

> 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>