| Message ID | ZzvZkMKhwmJESpdM@uudg.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | iommu/tegra241-cmdqv: do not call smp_processor_id in preemptible context | expand |
On 2024-11-18 21:19:28 [-0300], Luis Claudio R. Goncalves wrote: > With PREEMPT_RT enabled some of the calls to tegra241_cmdqv_get_cmdq() > during boot will happen in preemptible context. As this function calls > smp_processor_id(), these calls will trigger a "BUG: using smp_processor_id() > in preemptible" backtrace if DEBUG_PREEMPT is enabled. If this is only on PREEMPT_RT, where is the disabled preemption coming from on !PREEMPT_RT? > As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the > CPU number as a factor to balance out traffic on cmdq usage, it is safe > to use raw_smp_processor_id() here. > > Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com> Sebastian
On Tue, Nov 19, 2024 at 08:15:10AM +0100, Sebastian Andrzej Siewior wrote: > On 2024-11-18 21:19:28 [-0300], Luis Claudio R. Goncalves wrote: > > With PREEMPT_RT enabled some of the calls to tegra241_cmdqv_get_cmdq() > > during boot will happen in preemptible context. As this function calls > > smp_processor_id(), these calls will trigger a "BUG: using smp_processor_id() > > in preemptible" backtrace if DEBUG_PREEMPT is enabled. > > If this is only on PREEMPT_RT, where is the disabled preemption coming > from on !PREEMPT_RT? You are correct. This is not exclusive to PREEMPT_RT, also happens with the stock kernel. I ran more tests and this time I made sure DEBUG_PREEMPT was enabled. That was enough to reproduce the problem. > > As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the > > CPU number as a factor to balance out traffic on cmdq usage, it is safe > > to use raw_smp_processor_id() here. > > > > Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com> > > Sebastian > ---end quoted text---
diff --git a/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c b/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c index fcd13d301fff6..3ea4369e838fe 100644 --- a/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c +++ b/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c @@ -339,7 +339,7 @@ tegra241_cmdqv_get_cmdq(struct arm_smmu_device *smmu, * one CPU at a time can enter the process, while the others * will be spinning at the same lock. */ - lidx = smp_processor_id() % cmdqv->num_lvcmdqs_per_vintf; + lidx = raw_smp_processor_id() % cmdqv->num_lvcmdqs_per_vintf; vcmdq = vintf->lvcmdqs[lidx]; if (!vcmdq || !READ_ONCE(vcmdq->enabled)) return NULL;
With PREEMPT_RT enabled some of the calls to tegra241_cmdqv_get_cmdq() during boot will happen in preemptible context. As this function calls smp_processor_id(), these calls will trigger a "BUG: using smp_processor_id() in preemptible" backtrace if DEBUG_PREEMPT is enabled. As tegra241_cmdqv_get_cmdq() only calls smp_processor_id() to use the CPU number as a factor to balance out traffic on cmdq usage, it is safe to use raw_smp_processor_id() here. Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com> --- drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)