Re: [PATCH 1/1] iommu/vt-d: Fix suspicious RCU usage

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2025/2/20 15:21, Tian, Kevin wrote:
>> From: Lu Baolu<baolu.lu@linux.intel.com>
>> Sent: Tuesday, February 18, 2025 10:24 AM
>>
>> Commit <d74169ceb0d2> ("iommu/vt-d: Allocate DMAR fault interrupts
>> locally") moved the call to enable_drhd_fault_handling() to a code
>> path that does not hold any lock while traversing the drhd list. Fix
>> it by ensuring the dmar_global_lock lock is held when traversing the
>> drhd list.
>>
>> Without this fix, the following warning is triggered:
>>   =============================
>>   WARNING: suspicious RCU usage
>>   6.14.0-rc3 #55 Not tainted
>>   -----------------------------
>>   drivers/iommu/intel/dmar.c:2046 RCU-list traversed in non-reader section!!
>>                 other info that might help us debug this:
>>                 rcu_scheduler_active = 1, debug_locks = 1
>>   2 locks held by cpuhp/1/23:
>>   #0: ffffffff84a67c50 (cpu_hotplug_lock){++++}-{0:0}, at:
>> cpuhp_thread_fun+0x87/0x2c0
>>   #1: ffffffff84a6a380 (cpuhp_state-up){+.+.}-{0:0}, at:
>> cpuhp_thread_fun+0x87/0x2c0
>>   stack backtrace:
>>   CPU: 1 UID: 0 PID: 23 Comm: cpuhp/1 Not tainted 6.14.0-rc3 #55
>>   Call Trace:
>>    <TASK>
>>    dump_stack_lvl+0xb7/0xd0
>>    lockdep_rcu_suspicious+0x159/0x1f0
>>    ? __pfx_enable_drhd_fault_handling+0x10/0x10
>>    enable_drhd_fault_handling+0x151/0x180
>>    cpuhp_invoke_callback+0x1df/0x990
>>    cpuhp_thread_fun+0x1ea/0x2c0
>>    smpboot_thread_fn+0x1f5/0x2e0
>>    ? __pfx_smpboot_thread_fn+0x10/0x10
>>    kthread+0x12a/0x2d0
>>    ? __pfx_kthread+0x10/0x10
>>    ret_from_fork+0x4a/0x60
>>    ? __pfx_kthread+0x10/0x10
>>    ret_from_fork_asm+0x1a/0x30
>>    </TASK>
>>
>> Simply holding the lock in enable_drhd_fault_handling() will trigger a
>> lock order splat. Avoid holding the dmar_global_lock when calling
>> iommu_device_register(), which starts the device probe process.
> Can you elaborate the splat issue? It's not intuitive to me with a quick
> read of the code and iommu_device_register() is not occurred in above
> calling stack.

The lockdep splat looks like below:

  ======================================================
  WARNING: possible circular locking dependency detected
  6.14.0-rc3-00002-g8e4617b46db1 #57 Not tainted
  ------------------------------------------------------
  swapper/0/1 is trying to acquire lock:
  ffffffffa2a67c50 (cpu_hotplug_lock){++++}-{0:0}, at: 
iova_domain_init_rcaches.part.0+0x1d3/0x210

  but task is already holding lock:
  ffff9f4a87b171c8 (&domain->iova_cookie->mutex){+.+.}-{4:4}, at: 
iommu_dma_init_domain+0x122/0x2e0

  which lock already depends on the new lock.


  the existing dependency chain (in reverse order) is:

  -> #4 (&domain->iova_cookie->mutex){+.+.}-{4:4}:
         __lock_acquire+0x4a0/0xb50
         lock_acquire+0xd1/0x2e0
         __mutex_lock+0xa5/0xce0
         iommu_dma_init_domain+0x122/0x2e0
         iommu_setup_dma_ops+0x65/0xe0
         bus_iommu_probe+0x100/0x1d0
         iommu_device_register+0xd6/0x130
         intel_iommu_init+0x527/0x870
         pci_iommu_init+0x17/0x60
         do_one_initcall+0x7c/0x390
         do_initcalls+0xe8/0x1e0
         kernel_init_freeable+0x313/0x490
         kernel_init+0x24/0x240
         ret_from_fork+0x4a/0x60
         ret_from_fork_asm+0x1a/0x30

  -> #3 (&group->mutex){+.+.}-{4:4}:
         __lock_acquire+0x4a0/0xb50
         lock_acquire+0xd1/0x2e0
         __mutex_lock+0xa5/0xce0
         bus_iommu_probe+0x95/0x1d0
         iommu_device_register+0xd6/0x130
         intel_iommu_init+0x527/0x870
         pci_iommu_init+0x17/0x60
         do_one_initcall+0x7c/0x390
         do_initcalls+0xe8/0x1e0
         kernel_init_freeable+0x313/0x490
         kernel_init+0x24/0x240
         ret_from_fork+0x4a/0x60
         ret_from_fork_asm+0x1a/0x30

-> #2 (dmar_global_lock){++++}-{4:4}:
        __lock_acquire+0x4a0/0xb50
        lock_acquire+0xd1/0x2e0
        down_read+0x31/0x170
        enable_drhd_fault_handling+0x27/0x1a0
        cpuhp_invoke_callback+0x1e2/0x990
        cpuhp_issue_call+0xac/0x2c0
        __cpuhp_setup_state_cpuslocked+0x229/0x430
        __cpuhp_setup_state+0xc3/0x260
        irq_remap_enable_fault_handling+0x52/0x80
        apic_intr_mode_init+0x59/0xf0
        x86_late_time_init+0x29/0x50
        start_kernel+0x642/0x7f0
        x86_64_start_reservations+0x18/0x30
        x86_64_start_kernel+0x91/0xa0
        common_startup_64+0x13e/0x148

-> #1 (cpuhp_state_mutex){+.+.}-{4:4}:
        __lock_acquire+0x4a0/0xb50
        lock_acquire+0xd1/0x2e0
        __mutex_lock+0xa5/0xce0
        __cpuhp_setup_state_cpuslocked+0x81/0x430
        __cpuhp_setup_state+0xc3/0x260
        page_alloc_init_cpuhp+0x2d/0x40
        mm_core_init+0x1e/0x3a0
        start_kernel+0x277/0x7f0
        x86_64_start_reservations+0x18/0x30
        x86_64_start_kernel+0x91/0xa0
        common_startup_64+0x13e/0x148

-> #0 (cpu_hotplug_lock){++++}-{0:0}:
        check_prev_add+0xe2/0xc50
        validate_chain+0x57c/0x800
        __lock_acquire+0x4a0/0xb50
        lock_acquire+0xd1/0x2e0
        __cpuhp_state_add_instance+0x40/0x250
        iova_domain_init_rcaches.part.0+0x1d3/0x210
        iova_domain_init_rcaches+0x41/0x60
        iommu_dma_init_domain+0x1af/0x2e0
        iommu_setup_dma_ops+0x65/0xe0
        bus_iommu_probe+0x100/0x1d0
        iommu_device_register+0xd6/0x130
        intel_iommu_init+0x527/0x870
        pci_iommu_init+0x17/0x60
        do_one_initcall+0x7c/0x390
        do_initcalls+0xe8/0x1e0
        kernel_init_freeable+0x313/0x490
        kernel_init+0x24/0x240
        ret_from_fork+0x4a/0x60
        ret_from_fork_asm+0x1a/0x30

  other info that might help us debug this:

  Chain exists of:
    cpu_hotplug_lock --> &group->mutex --> &domain->iova_cookie->mutex

   Possible unsafe locking scenario:

         CPU0                    CPU1
         ----                    ----
    lock(&domain->iova_cookie->mutex);
                                 lock(&group->mutex);
                                 lock(&domain->iova_cookie->mutex);
    rlock(cpu_hotplug_lock);

   *** DEADLOCK ***

  3 locks held by swapper/0/1:
   #0: ffffffffa6442ab0 (dmar_global_lock){++++}-{4:4}, at: 
intel_iommu_init+0x42c/0x87
   #1: ffff9f4a87b11310 (&group->mutex){+.+.}-{4:4}, at: 
bus_iommu_probe+0x95/0x1d0
   #2: ffff9f4a87b171c8 (&domain->iova_cookie->mutex){+.+.}-{4:4}, at: 
iommu_dma_init_d

  stack backtrace:
  CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 
6.14.0-rc3-00002-g8e4617b46db1 #57
  Call Trace:
   <TASK>
   dump_stack_lvl+0x93/0xd0
   print_circular_bug+0x133/0x1c0
   check_noncircular+0x12c/0x150
   check_prev_add+0xe2/0xc50
   ? add_chain_cache+0x108/0x460
   validate_chain+0x57c/0x800
   __lock_acquire+0x4a0/0xb50
   lock_acquire+0xd1/0x2e0
   ? iova_domain_init_rcaches.part.0+0x1d3/0x210
   ? rcu_is_watching+0x11/0x50
   __cpuhp_state_add_instance+0x40/0x250
   ? iova_domain_init_rcaches.part.0+0x1d3/0x210
   iova_domain_init_rcaches.part.0+0x1d3/0x210
   iova_domain_init_rcaches+0x41/0x60
   iommu_dma_init_domain+0x1af/0x2e0
   iommu_setup_dma_ops+0x65/0xe0
   bus_iommu_probe+0x100/0x1d0
   iommu_device_register+0xd6/0x130
   intel_iommu_init+0x527/0x870
   ? __pfx_pci_iommu_init+0x10/0x10
   pci_iommu_init+0x17/0x60
   do_one_initcall+0x7c/0x390
   do_initcalls+0xe8/0x1e0
   kernel_init_freeable+0x313/0x490
   ? __pfx_kernel_init+0x10/0x10
   kernel_init+0x24/0x240
   ? _raw_spin_unlock_irq+0x33/0x50
   ret_from_fork+0x4a/0x60
   ? __pfx_kernel_init+0x10/0x10
   ret_from_fork_asm+0x1a/0x30
   </TASK>

Thanks,
baolu