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

[Karl Mehltretter <kmehltretter@gmail.com>]

On Sat, Nov 07, 2026 at 10:13:51PM +0100, John Paul Adrian Glaubitz wrote:
> 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?
> 

Hi Adrian,

Yes. It needs PREEMPT=y and DEBUG_PREEMPT=y. Debian's sparc64 kernel
uses PREEMPT_VOLUNTARY, so a kernel built from that config cannot
print it.

sparc64_defconfig with PREEMPT, DEBUG_PREEMPT and lockdep, booted with

  qemu-system-sparc64 -M sun4u -m 512 -nographic -no-reboot \
      -kernel arch/sparc/boot/image \
      -append "console=ttyS0 loglevel=7 panic=-1"

prints three reports on every boot, from the pata_cmd64x and sunhme
probes.

Danish Khateeb saw the same reports:
https://lore.kernel.org/r/20260923144909.414968-1-danishkhateeb03@gmail.com/

It is only a warning. The per-CPU value only picks the first pool to
try, and each pool has its own lock.

Thanks,
Karl