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

[Bartosz Golaszewski]

On Tue, 29 Aug 2023 at 09:59, Krzysztof Kozlowski
<krzysztof.kozlowski@xxxxxxxxxx> wrote:
>
> 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.
>

Hmm, I had the list_add() implementation in mind as well as examples
from https://www.kernel.org/doc/Documentation/memory-barriers.txt and
was under the impression that WRITE_ONCE() here is enough. I need to
double check it.

Bart