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SubjectRe: [PATCH v2 5/7] iommu/vt-d: Fix suspicious RCU usage in probe_acpi_namespace_devices()
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On 2019-06-12 01:28, Lu Baolu wrote:
> The drhd and device scope list should be iterated with the
> iommu global lock held. Otherwise, a suspicious RCU usage
> message will be displayed.
>
> [ 3.695886] =============================
> [ 3.695917] WARNING: suspicious RCU usage
> [ 3.695950] 5.2.0-rc2+ #2467 Not tainted
> [ 3.695981] -----------------------------
> [ 3.696014] drivers/iommu/intel-iommu.c:4569 suspicious rcu_dereference_check() usage!
> [ 3.696069]
> other info that might help us debug this:
>
> [ 3.696126]
> rcu_scheduler_active = 2, debug_locks = 1
> [ 3.696173] no locks held by swapper/0/1.
> [ 3.696204]
> stack backtrace:
> [ 3.696241] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.2.0-rc2+ #2467
> [ 3.696370] Call Trace:
> [ 3.696404] dump_stack+0x85/0xcb
> [ 3.696441] intel_iommu_init+0x128c/0x13ce
> [ 3.696478] ? kmem_cache_free+0x16b/0x2c0
> [ 3.696516] ? __fput+0x14b/0x270
> [ 3.696550] ? __call_rcu+0xb7/0x300
> [ 3.696583] ? get_max_files+0x10/0x10
> [ 3.696631] ? set_debug_rodata+0x11/0x11
> [ 3.696668] ? e820__memblock_setup+0x60/0x60
> [ 3.696704] ? pci_iommu_init+0x16/0x3f
> [ 3.696737] ? set_debug_rodata+0x11/0x11
> [ 3.696770] pci_iommu_init+0x16/0x3f
> [ 3.696805] do_one_initcall+0x5d/0x2e4
> [ 3.696844] ? set_debug_rodata+0x11/0x11
> [ 3.696880] ? rcu_read_lock_sched_held+0x6b/0x80
> [ 3.696924] kernel_init_freeable+0x1f0/0x27c
> [ 3.696961] ? rest_init+0x260/0x260
> [ 3.696997] kernel_init+0xa/0x110
> [ 3.697028] ret_from_fork+0x3a/0x50
>
> Fixes: fa212a97f3a36 ("iommu/vt-d: Probe DMA-capable ACPI name space devices")
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
> drivers/iommu/intel-iommu.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index 19c4c387a3f6..84e650c6a46d 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -4793,8 +4793,10 @@ int __init intel_iommu_init(void)
> cpuhp_setup_state(CPUHP_IOMMU_INTEL_DEAD, "iommu/intel:dead", NULL,
> intel_iommu_cpu_dead);
>
> + down_read(&dmar_global_lock);
> if (probe_acpi_namespace_devices())
> pr_warn("ACPI name space devices didn't probe correctly\n");
> + up_read(&dmar_global_lock);

Doing a bit of archaeology here, is this actually broken? If any ANDD
entries exist, we'd end up doing:

down_read(&dmar_global_lock)
probe_acpi_namespace_devices()
-> iommu_probe_device()
-> iommu_create_device_direct_mappings()
-> iommu_get_resv_regions()
-> intel_iommu_get_resv_regions()
-> down_read(&dmar_global_lock)

I'm wondering whether this might explain why my bus_set_iommu series
prevented Baolu's machine from booting, since "iommu: Move bus setup to
IOMMU device registration" creates the same condition where we end up in
get_resv_regions (via bus_iommu_probe() this time) from the same task
that already holds dmar_global_lock. Of course that leaves me wondering
how it *did* manage to boot OK on my Xeon box, but maybe there's a
config difference or dumb luck at play?

Robin.

>
> /* Finally, we enable the DMA remapping hardware. */
> for_each_iommu(iommu, drhd) {

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