Re: [PATCH 03/11] firmware: qcom-scm: atomically assign and read the global __scm pointer

[Krzysztof Kozlowski]

On 28/08/2023 21:24, Bartosz Golaszewski wrote:
> Checking for the availability of SCM bridge can happen from any context.
> It's only by chance that we haven't run into concurrency issues but with
> the upcoming SHM Bridge driver that will be initiated at the same
> initcall level, we need to assure the assignment and readback of the
> __scm pointer is atomic.
> 
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> ---
>  drivers/firmware/qcom_scm.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/firmware/qcom_scm.c b/drivers/firmware/qcom_scm.c
> index 980fcfa20b9f..422de70faff8 100644
> --- a/drivers/firmware/qcom_scm.c
> +++ b/drivers/firmware/qcom_scm.c
> @@ -1331,7 +1331,7 @@ static int qcom_scm_find_dload_address(struct device *dev, u64 *addr)
>   */
>  bool qcom_scm_is_available(void)
>  {
> -	return !!__scm;
> +	return !!READ_ONCE(__scm);
>  }
>  EXPORT_SYMBOL(qcom_scm_is_available);
>  
> @@ -1477,8 +1477,8 @@ static int qcom_scm_probe(struct platform_device *pdev)
>  	if (ret)
>  		return ret;
>  
> -	__scm = scm;
> -	__scm->dev = &pdev->dev;
> +	scm->dev = &pdev->dev;
> +	WRITE_ONCE(__scm, scm);

Your re-ordering is not effective here, I think. I don't understand it's
purpose exactly, but scm->dev assignment is not WRITE_ONCE(), thus it
can be reordered:

"compiler is also forbidden from reordering successive instances of
READ_ONCE and WRITE_ONCE" <- so compiler is not forbidden to reorder
other stuff.

"Ensuring that the compiler does not fold, spindle, or otherwise
mutilate accesses that either do not require ordering or that interact"
<- which means you do not require ordering here.

Best regards,
Krzysztof