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

[Baolu Lu]

On 2025/2/21 15:22, Tian, Kevin wrote:
> > From: Baolu Lu<baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Thursday, February 20, 2025 7:38 PM
> >
> > On 2025/2/20 15:21, Tian, Kevin wrote:
> > > > From: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > 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:
> Thanks and it's clear now. Probably you can expand "to avoid unnecessary
> lock order splat " a little bit to mark the dead lock between dmar_global_lock
> and cpu_hotplug_lock (acquired in path of iommu_device_register()).

Yes, sure.