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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.14.0 Subject: Re: [PATCH 03/11] firmware: qcom-scm: atomically assign and read the global __scm pointer Content-Language: en-US To: Bartosz Golaszewski , Andy Gross , Bjorn Andersson , Konrad Dybcio , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Catalin Marinas , Will Deacon , Arnd Bergmann , Alex Elder , Srini Kandagatla Cc: kernel@quicinc.com, linux-arm-msm@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20230828192507.117334-1-bartosz.golaszewski@linaro.org> <20230828192507.117334-4-bartosz.golaszewski@linaro.org> From: Krzysztof Kozlowski In-Reply-To: <20230828192507.117334-4-bartosz.golaszewski@linaro.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org 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 > --- > 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