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

[Jesse Taube]

On Tue, Jun 16, 2026 at 11:41 PM Nam Cao <namcao@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-riscv/20260612-vec_unaligned_drop_init-v1-1-df969210ae34@xxxxxxxxxxxxxxxxxxx/
> Fixes: a00e022be531 ("riscv: Annotate unaligned access init functions")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>

Acked-by: Jesse Taube <jtaubepe@xxxxxxxxxx>

> ---
>  arch/riscv/kernel/unaligned_access_speed.c | 19 ++-----------------
>  1 file changed, 2 insertions(+), 17 deletions(-)
>
> diff --git a/arch/riscv/kernel/unaligned_access_speed.c b/arch/riscv/kernel/unaligned_access_speed.c
> index bb57eb5d19df..6e35bca568de 100644
> --- a/arch/riscv/kernel/unaligned_access_speed.c
> +++ b/arch/riscv/kernel/unaligned_access_speed.c
> @@ -6,7 +6,6 @@
>  #include <linux/cpu.h>
>  #include <linux/cpumask.h>
>  #include <linux/jump_label.h>
> -#include <linux/kthread.h>
>  #include <linux/mm.h>
>  #include <linux/smp.h>
>  #include <linux/types.h>
> @@ -288,18 +287,9 @@ static void check_vector_unaligned_access(struct work_struct *work __always_unus
>         __free_pages(page, MISALIGNED_BUFFER_ORDER);
>  }
>
> -/* Measure unaligned access speed on all CPUs present at boot in parallel. */
> -static int __init vec_check_unaligned_access_speed_all_cpus(void *unused __always_unused)
> -{
> -       schedule_on_each_cpu(check_vector_unaligned_access);
> -       riscv_hwprobe_complete_async_probe();
> -
> -       return 0;
> -}
>  #else /* CONFIG_RISCV_PROBE_VECTOR_UNALIGNED_ACCESS */
> -static int __init vec_check_unaligned_access_speed_all_cpus(void *unused __always_unused)
> +static void check_vector_unaligned_access(struct work_struct *work __always_unused)
>  {
> -       return 0;
>  }
>  #endif
>
> @@ -387,12 +377,7 @@ static int __init check_unaligned_access_all_cpus(void)
>                         per_cpu(vector_misaligned_access, cpu) = unaligned_vector_speed_param;
>         } else if (!check_vector_unaligned_access_emulated_all_cpus() &&
>                    IS_ENABLED(CONFIG_RISCV_PROBE_VECTOR_UNALIGNED_ACCESS)) {
> -               riscv_hwprobe_register_async_probe();
> -               if (IS_ERR(kthread_run(vec_check_unaligned_access_speed_all_cpus,
> -                                      NULL, "vec_check_unaligned_access_speed_all_cpus"))) {
> -                       pr_warn("Failed to create vec_unalign_check kthread\n");
> -                       riscv_hwprobe_complete_async_probe();
> -               }
> +               schedule_on_each_cpu(check_vector_unaligned_access);
>         }
>
>         /*
> --
> 2.47.3
>
>
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