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[2a02:3100:b149:1401:90c6:65b4:35d:1e3d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b7efb134ccsm103722785e9.10.2026.11.07.17.35.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 07 Nov 2026 17:35:52 -0800 (PST) Date: Sun, 8 Nov 2026 02:35:49 +0100 From: Karl Mehltretter To: John Paul Adrian Glaubitz Cc: "David S. Miller" , Andreas Larsson , sparclinux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sparc/iommu: Use raw_cpu_read() for iommu_hash_common in iommu_tbl_range_alloc() Message-ID: References: <20261107195147.70084-1-kmehltretter@gmail.com> <155ec5c6c5f6d4287a420191cb3927eba1d97526.camel@physik.fu-berlin.de> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <155ec5c6c5f6d4287a420191cb3927eba1d97526.camel@physik.fu-berlin.de> 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 > > --- > > 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