Re: [PATCH] iommu/vt-d: Remove unnecessary locking in intel_irq_remapping_alloc()

Baolu Lu
2023-03-15T01:15:59Z

On 3/14/23 11:54 PM, Jacob Pan wrote:
> Hi BaoLu,
>
> On Tue, 14 Mar 2023 13:18:36 +0800, Lu Baolu <baolu.lu@linux.intel.com>
> wrote:
>
>> The global rwsem dmar_global_lock was introduced by commit 3a5670e8ac932
>> ("iommu/vt-d: Introduce a rwsem to protect global data structures"). It
>> is used to protect DMAR related global data from DMAR hotplug operations.
>>
>> Using dmar_global_lock in intel_irq_remapping_alloc() is unnecessary as
>> the DMAR global data structures are not touched there. Remove it to avoid
>> below lockdep warning.
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 6.3.0-rc2 #468 Not tainted
>> ------------------------------------------------------
>> swapper/0/1 is trying to acquire lock:
>> ff1db4cb40178698 (&domain->mutex){+.+.}-{3:3},
>> at: __irq_domain_alloc_irqs+0x3b/0xa0
>>
>> but task is already holding lock:
>> ffffffffa0c1cdf0 (dmar_global_lock){++++}-{3:3},
>> at: intel_iommu_init+0x58e/0x880
>>
>> which lock already depends on the new lock.
>>
>> the existing dependency chain (in reverse order) is:
>>
>> -> #1 (dmar_global_lock){++++}-{3:3}:
>> lock_acquire+0xd6/0x320
>> down_read+0x42/0x180
>> intel_irq_remapping_alloc+0xad/0x750
>> mp_irqdomain_alloc+0xb8/0x2b0
>> irq_domain_alloc_irqs_locked+0x12f/0x2d0
>> __irq_domain_alloc_irqs+0x56/0xa0
>> alloc_isa_irq_from_domain.isra.7+0xa0/0xe0
>> mp_map_pin_to_irq+0x1dc/0x330
>> setup_IO_APIC+0x128/0x210
>> apic_intr_mode_init+0x67/0x110
>> x86_late_time_init+0x24/0x40
>> start_kernel+0x41e/0x7e0
>> secondary_startup_64_no_verify+0xe0/0xeb
>>
>> -> #0 (&domain->mutex){+.+.}-{3:3}:
>> check_prevs_add+0x160/0xef0
>> __lock_acquire+0x147d/0x1950
>> lock_acquire+0xd6/0x320
>> __mutex_lock+0x9c/0xfc0
>> __irq_domain_alloc_irqs+0x3b/0xa0
>> dmar_alloc_hwirq+0x9e/0x120
>> iommu_pmu_register+0x11d/0x200
>> intel_iommu_init+0x5de/0x880
>> pci_iommu_init+0x12/0x40
>> do_one_initcall+0x65/0x350
>> kernel_init_freeable+0x3ca/0x610
>> kernel_init+0x1a/0x140
>> ret_from_fork+0x29/0x50
>>
>> other info that might help us debug this:
>>
>> Possible unsafe locking scenario:
>>
>> CPU0 CPU1
>> ---- ----
>> lock(dmar_global_lock);
>> lock(&domain->mutex);
>> lock(dmar_global_lock);
>> lock(&domain->mutex);
>>
>> *** DEADLOCK ***
>>
>> Fixes: 9dbb8e3452ab ("irqdomain: Switch to per-domain locking")
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>> drivers/iommu/intel/irq_remapping.c | 6 ------
>> 1 file changed, 6 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/irq_remapping.c
>> b/drivers/iommu/intel/irq_remapping.c index 6d01fa078c36..df9e261af0b5
>> 100644 --- a/drivers/iommu/intel/irq_remapping.c
>> +++ b/drivers/iommu/intel/irq_remapping.c
>> @@ -311,14 +311,12 @@ static int set_ioapic_sid(struct irte *irte, int
>> apic) if (!irte)
>> return -1;
>>
>> - down_read(&dmar_global_lock);
>> for (i = 0; i < MAX_IO_APICS; i++) {
>> if (ir_ioapic[i].iommu && ir_ioapic[i].id == apic) {
>> sid = (ir_ioapic[i].bus << 8) |
>> ir_ioapic[i].devfn; break;
>> }
>> }
>> - up_read(&dmar_global_lock);
>>
>> if (sid == 0) {
>> pr_warn("Failed to set source-id of IOAPIC (%d)\n",
>> apic); @@ -338,14 +336,12 @@ static int set_hpet_sid(struct irte *irte,
>> u8 id) if (!irte)
>> return -1;
>>
>> - down_read(&dmar_global_lock);
>> for (i = 0; i < MAX_HPET_TBS; i++) {
>> if (ir_hpet[i].iommu && ir_hpet[i].id == id) {
>> sid = (ir_hpet[i].bus << 8) | ir_hpet[i].devfn;
>> break;
>> }
>> }
>> - up_read(&dmar_global_lock);
>>
>> if (sid == 0) {
>> pr_warn("Failed to set source-id of HPET block (%d)\n",
>> id); @@ -1339,9 +1335,7 @@ static int intel_irq_remapping_alloc(struct
>> irq_domain *domain, if (!data)
>> goto out_free_parent;
>>
>> - down_read(&dmar_global_lock);
>> index = alloc_irte(iommu, &data->irq_2_iommu, nr_irqs);
>> - up_read(&dmar_global_lock);
>> if (index < 0) {
>> pr_warn("Failed to allocate IRTE\n");
>> kfree(data);
> Reviewed-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
>
> slightly beyond the scope of this, do we need to take dmar_global_lock
> below? shouldn't it be in single threaded context?
>
> down_write(&dmar_global_lock);
> ret = dmar_dev_scope_init();
> up_write(&dmar_global_lock);
>
> return ret;
> }
> rootfs_initcall(ir_dev_scope_init);

Yes, agreed. This runs in a single threaded context. I will remove the
locking in a cleanup patch.

Best regards,
baolu