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Tue, 19 Nov 2024 10:33:36 -0800 (PST) From: Krzysztof Kozlowski Date: Tue, 19 Nov 2024 19:33:17 +0100 Subject: [PATCH 1/6] firmware: qcom: scm: Fix missing read barrier in qcom_scm_is_available() Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20241119-qcom-scm-missing-barriers-and-all-sort-of-srap-v1-1-7056127007a7@linaro.org> References: <20241119-qcom-scm-missing-barriers-and-all-sort-of-srap-v1-0-7056127007a7@linaro.org> In-Reply-To: <20241119-qcom-scm-missing-barriers-and-all-sort-of-srap-v1-0-7056127007a7@linaro.org> To: Bjorn Andersson , Konrad Dybcio , Mukesh Ojha , Dmitry Baryshkov , Stephan Gerhold , Bartosz Golaszewski , Kuldeep Singh , Elliot Berman , Andrew Halaney , Avaneesh Kumar Dwivedi , Andy Gross Cc: linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Krzysztof Kozlowski X-Mailer: b4 0.14.2 X-Developer-Signature: v=1; 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a=openpgp; fpr=9BD07E0E0C51F8D59677B7541B93437D3B41629B 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 v= ariable initialization") Cc: Signed-off-by: Krzysztof Kozlowski Reviewed-by: Bartosz Golaszewski --- 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..246d672e8f7f0e2a326a03a5af4= 0cd434a665e67 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); =20 @@ -2024,7 +2025,7 @@ static int qcom_scm_probe(struct platform_device *pde= v) if (ret) return ret; =20 - /* Let all above stores be available after this */ + /* Paired with smp_load_acquire() in qcom_scm_is_available(). */ smp_store_release(&__scm, scm); =20 irq =3D platform_get_irq_optional(pdev, 0); --=20 2.43.0