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.

Fixes: d74169ceb0d2 ("iommu/vt-d: Allocate DMAR fault interrupts locally") Reported-by: Ido Schimmel idosch@idosch.org Closes: https://lore.kernel.org/linux-iommu/Zx9OwdLIc_VoQ0- a@shredder.mtl.com/ Cc: stable@vger.kernel.org Signed-off-by: Lu Baolu baolu.lu@linux.intel.com

---

drivers/iommu/intel/dmar.c | 1 + drivers/iommu/intel/iommu.c | 7 +++++++ 2 files changed, 8 insertions(+)

diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c index 9f424acf474e..e540092d664d 100644 --- a/drivers/iommu/intel/dmar.c +++ b/drivers/iommu/intel/dmar.c @@ -2043,6 +2043,7 @@ int enable_drhd_fault_handling(unsigned int cpu) /* * Enable fault control interrupt. */

• guard(rwsem_read)(&dmar_global_lock); for_each_iommu(iommu, drhd) { u32 fault_status; int ret;

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c index cc46098f875b..9a1e61b429ca 100644 --- a/drivers/iommu/intel/iommu.c +++ b/drivers/iommu/intel/iommu.c @@ -3146,7 +3146,14 @@ int __init intel_iommu_init(void) iommu_device_sysfs_add(&iommu->iommu, NULL, intel_iommu_groups, "%s", iommu->name);

• /*
  

•  * The iommu device probe is protected by the
  

iommu_probe_device_lock.

•  * Release the dmar_global_lock before entering the device
  

probe path

•  * to avoid unnecessary lock order splat.
  

•  */
  

• up_read(&dmar_global_lock);
  

  iommu_device_register(&iommu->iommu,

&intel_iommu_ops, NULL);

• down_read(&dmar_global_lock);
  
  

  iommu_pmu_register(iommu); }

-- 2.43.0