On Tue, Nov 19, 2024 at 7:37 PM Krzysztof Kozlowski krzysztof.kozlowski@linaro.org wrote:
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 Signed-off-by: Krzysztof Kozlowski krzysztof.kozlowski@linaro.org
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..246d672e8f7f0e2a326a03a5af40cd434a665e67 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);
@@ -2024,7 +2025,7 @@ 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);-- 2.43.0
I'm not an expert on barriers and SMP but the explanation sounds correct to me.
Reviewed-by: Bartosz Golaszewski bartosz.golaszewski@linaro.org