[PATCH] sparc/iommu: Use raw_cpu_read() for iommu_hash_common in iommu_tbl_range_alloc()

[PATCH] sparc/iommu: Use raw_cpu_read() for iommu_hash_common in iommu_tbl_range_alloc()

[Karl Mehltretter]

iommu_tbl_range_alloc() reads the per-CPU starting pool cookie with
__this_cpu_read() before acquiring pool->lock. When called from
preemptible DMA allocation paths with PREEMPT and DEBUG_PREEMPT enabled,
check_preemption_disabled() reports a bug.

The per-CPU cookie only distributes allocations across pools, and each
pool is synchronized by pool->lock. Replace __this_cpu_read() with
raw_cpu_read().

Fixes: ff7d37a50202 ("Break up monolithic iommu table/lock into finer graularity pools and lock")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Assisted by an LLM.

Tested on v7.4-rc1 (d48723711bae) under QEMU sparc64 (sun4u, -smp 2)
with PREEMPT and DEBUG_PREEMPT. Without this patch, two consecutive
boots emit three "BUG: using __this_cpu_read() in preemptible
[00000000] code: swapper/0/1" diagnostics in iommu_tbl_range_alloc().
With this patch, two consecutive boots complete without the
__this_cpu_read() preemption diagnostic.

 arch/sparc/kernel/iommu-common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/sparc/kernel/iommu-common.c b/arch/sparc/kernel/iommu-common.c
index 23ca75f092770..014d4e9283d55 100644
--- a/arch/sparc/kernel/iommu-common.c
+++ b/arch/sparc/kernel/iommu-common.c
@@ -101,7 +101,7 @@ unsigned long iommu_tbl_range_alloc(struct device *dev,
 				unsigned long mask,
 				unsigned int align_order)
 {
-	unsigned int pool_hash = __this_cpu_read(iommu_hash_common);
+	unsigned int pool_hash = raw_cpu_read(iommu_hash_common);
 	unsigned long n, end, start, limit, boundary_size;
 	struct iommu_pool *pool;
 	int pass = 0;
base-commit: d48723711bae9b8cdc56719c448ba42fef28cbee

-- 
2.39.5 (Apple Git-154)

Re: [PATCH] sparc/iommu: Use raw_cpu_read() for iommu_hash_common in iommu_tbl_range_alloc()

[John Paul Adrian Glaubitz]

Hi Karl,

On Sat, 2026-11-07 at 20:51 +0100, Karl Mehltretter wrote:
> Fixes: ff7d37a50202 ("Break up monolithic iommu table/lock into finer graularity pools and lock")
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
> Assisted by an LLM.
> 
> Tested on v7.4-rc1 (d48723711bae) under QEMU sparc64 (sun4u, -smp 2)
> with PREEMPT and DEBUG_PREEMPT. Without this patch, two consecutive
> boots emit three "BUG: using __this_cpu_read() in preemptible
> [00000000] code: swapper/0/1" diagnostics in iommu_tbl_range_alloc().
> With this patch, two consecutive boots complete without the
> __this_cpu_read() preemption diagnostic.

According to your Fixes tag, this bug was introduced in 2015. However,
I have to admit I have never seen this message, neither on sun4u nor
on sun4v machines.

Is there a reliable way to reproduce this bug?

Adrian

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