6.6-stable review patch. If anyone has any objections, please let me know.
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From: Krzysztof Kozlowski krzysztof.kozlowski@linaro.org
[ Upstream commit 0a744cceebd0480cb39587b3b1339d66a9d14063 ]
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 Reviewed-by: Bartosz Golaszewski bartosz.golaszewski@linaro.org Signed-off-by: Krzysztof Kozlowski krzysztof.kozlowski@linaro.org Link: https://lore.kernel.org/r/20241209-qcom-scm-missing-barriers-and-all-sort-of... Signed-off-by: Bjorn Andersson andersson@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/firmware/qcom_scm.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/drivers/firmware/qcom_scm.c b/drivers/firmware/qcom_scm.c index 7af59985f1c1f..4c5c2b73d42c2 100644 --- a/drivers/firmware/qcom_scm.c +++ b/drivers/firmware/qcom_scm.c @@ -1339,7 +1339,8 @@ static int qcom_scm_find_dload_address(struct device *dev, u64 *addr) */ 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);
@@ -1457,7 +1458,7 @@ 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);