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

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Message-ID: <Z7RdnR2onJ2AZIJl@shredder>
Date: Tue, 18 Feb 2025 12:14:53 +0200
From: Ido Schimmel <idosch@...sch.org>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	leitao@...ian.org
Subject: Re: [PATCH 1/1] iommu/vt-d: Fix suspicious RCU usage

+ Breno who also encountered this issue

On Tue, Feb 18, 2025 at 10:24:21AM +0800, Lu Baolu wrote:
> 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.
> 
> Fixes: d74169ceb0d2 ("iommu/vt-d: Allocate DMAR fault interrupts locally")
> Reported-by: Ido Schimmel <idosch@...sch.org>
> Closes: https://lore.kernel.org/linux-iommu/Zx9OwdLIc_VoQ0-a@shredder.mtl.com/
> Cc: stable@...r.kernel.org
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>

Thanks for the fix. I tested it and the warning is gone.

Tested-by: Ido Schimmel <idosch@...dia.com>