Re: [PATCH 1/6] firmware: qcom: scm: Fix missing read barrier in qcom_scm_is_available()

[Bartosz Golaszewski <brgl@bgdev.pl>]

On Tue, Nov 19, 2024 at 7:37 PM Krzysztof Kozlowski
<krzysztof.kozlowski@linaro.org> wrote:
>
> Commit 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq
> completion variable initialization") introduced a write barrier in probe
> function to store global '__scm' variable.  It also claimed that it
> added a read barrier, because as we all known barriers are paired (see
> memory-barriers.txt: "Note that write barriers should normally be paired
> with read or address-dependency barriers"), however it did not really
> add it.
>
> The offending commit used READ_ONCE() to access '__scm' global which is
> not a barrier.
>
> The barrier is needed so the store to '__scm' will be properly visible.
> This is most likely not fatal in current driver design, because missing
> read barrier would mean qcom_scm_is_available() callers will access old
> value, NULL.  Driver does not support unbinding and does not correctly
> handle probe failures, thus there is no risk of stale or old pointer in
> '__scm' variable.
>
> However for code correctness, readability and to be sure that we did not
> mess up something in this tricky topic of SMP barriers, add a read
> barrier for accessing '__scm'.  Change also comment from useless/obvious
> what does barrier do, to what is expected: which other parts of the code
> are involved here.
>
> Fixes: 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq completion variable initialization")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> ---
>  drivers/firmware/qcom/qcom_scm.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> index 72bf87ddcd969834609cda2aa915b67505e93943..246d672e8f7f0e2a326a03a5af40cd434a665e67 100644
> --- a/drivers/firmware/qcom/qcom_scm.c
> +++ b/drivers/firmware/qcom/qcom_scm.c
> @@ -1867,7 +1867,8 @@ static int qcom_scm_qseecom_init(struct qcom_scm *scm)
>   */
>  bool qcom_scm_is_available(void)
>  {
> -       return !!READ_ONCE(__scm);
> +       /* Paired with smp_store_release() in qcom_scm_probe */
> +       return !!smp_load_acquire(&__scm);
>  }
>  EXPORT_SYMBOL_GPL(qcom_scm_is_available);
>
> @@ -2024,7 +2025,7 @@ static int qcom_scm_probe(struct platform_device *pdev)
>         if (ret)
>                 return ret;
>
> -       /* Let all above stores be available after this */
> +       /* Paired with smp_load_acquire() in qcom_scm_is_available(). */
>         smp_store_release(&__scm, scm);
>
>         irq = platform_get_irq_optional(pdev, 0);
>
> --
> 2.43.0
>
>

I'm not an expert on barriers and SMP but the explanation sounds correct to me.

Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>