Re: [PATCH v3 01/15] iommu/vt-d: Handle race between registration and device probe

From: Baolu Lu <baolu.lu@linux.intel.com>
To: Robin Murphy <robin.murphy@arm.com>, joro@8bytes.org
Cc: baolu.lu@linux.intel.com, will@kernel.org, iommu@lists.linux.dev,
	linux-arm-kernel@lists.infradead.org,
	suravee.suthikulpanit@amd.com, vasant.hegde@amd.com,
	mjrosato@linux.ibm.com, gerald.schaefer@linux.ibm.com,
	schnelle@linux.ibm.com, linux-s390@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 01/15] iommu/vt-d: Handle race between registration and device probe
Date: Fri, 8 Jul 2022 15:52:12 +0800	[thread overview]
Message-ID: <903a1677-4217-d793-6295-3927143d98fb@linux.intel.com> (raw)
In-Reply-To: <894db0ccae854b35c73814485569b634237b5538.1657034828.git.robin.murphy@arm.com>

On 2022/7/6 01:08, Robin Murphy wrote:
> That also highlights an issue with intel_iommu_get_resv_regions() taking
> dmar_global_lock from within a section where intel_iommu_init() already
> holds it, which already exists via probe_acpi_namespace_devices() when
> an ANDD device is probed, but gets more obvious with the upcoming change
> to iommu_device_register(). Since they are both read locks it manages
> not to deadlock in practice, so I'm leaving it here for someone with
> more confidence to tackle a larger rework of the locking.

I am trying to reproduce this problem. Strangely, even if I selected
CONFIG_LOCKDEP=y, the kernel didn't complain anything. :-)

In fact the rmrr list in the Intel IOMMU driver is always static after
parsing the ACPI/DMAR tables. There's no need to protect it with a lock.
Hence we can safely remove below down/up_read().

4512 static void intel_iommu_get_resv_regions(struct device *device,
4513                                          struct list_head *head)
4514 {
4515         int prot = DMA_PTE_READ | DMA_PTE_WRITE;
4516         struct iommu_resv_region *reg;
4517         struct dmar_rmrr_unit *rmrr;
4518         struct device *i_dev;
4519         int i;
4520
4521         down_read(&dmar_global_lock);
4522         for_each_rmrr_units(rmrr) {
4523                 for_each_active_dev_scope(rmrr->devices, 
rmrr->devices_cnt,
4524                                           i, i_dev) {

Best regards,
baolu