Re: [PATCH v2] acpi/hmat: Fix lockdep warning for hmem_register_resource()

[Dave Jiang <dave.jiang@intel.com>]

On 10/15/25 1:19 PM, dan.j.williams@intel.com wrote:
> Dave Jiang wrote:
>> The following lockdep splat was observed while kernel auto-online a CXL
>> memory region:
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 6.17.0djtest+ #53 Tainted: G        W
>> ------------------------------------------------------
>> systemd-udevd/3334 is trying to acquire lock:
>> ffffffff90346188 (hmem_resource_lock){+.+.}-{4:4}, at: hmem_register_resource+0x31/0x50
>>
>> but task is already holding lock:
>> ffffffff90338890 ((node_chain).rwsem){++++}-{4:4}, at: blocking_notifier_call_chain+0x2e/0x70
>>
>> which lock already depends on the new lock.
>> [..]
>> Chain exists of:
>>   hmem_resource_lock --> mem_hotplug_lock --> (node_chain).rwsem
>>
>>  Possible unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   rlock((node_chain).rwsem);
>>                                lock(mem_hotplug_lock);
>>                                lock((node_chain).rwsem);
>>   lock(hmem_resource_lock);
>>
>> The lock ordering can cause potential deadlock. There are instances
>> where hmem_resource_lock is taken after (node_chain).rwsem, and vice
>> versa.
>>
>> Remove registering of target devices from the hmat_callback(). By the
>> time the hmat hotplug notifier is being called, there should not be
>> hmem targets that still need to be registered.
>>
>> Fixes: cf8741ac57ed ("ACPI: NUMA: HMAT: Register "soft reserved" memory as an "hmem" device")
>> Link: https://lore.kernel.org/nvdimm/68e46a09c2a07_2980100f3@dwillia2-mobl4.notmuch/
>> Suggested-by: Dan Williams <dan.j.williams@intel.com>
>> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
>> ---
>> v2:
>> - Drop target registering in hmat_callback instead. (Dan)
>> ---
>>  drivers/acpi/numa/hmat.c | 10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
>> index 5a36d57289b4..5084ae1688f6 100644
>> --- a/drivers/acpi/numa/hmat.c
>> +++ b/drivers/acpi/numa/hmat.c
>> @@ -874,7 +874,8 @@ static void hmat_register_target_devices(struct memory_target *target)
>>  	}
>>  }
>>  
>> -static void hmat_register_target(struct memory_target *target)
>> +static void hmat_register_target(struct memory_target *target,
>> +				 bool register_devices)
>>  {
>>  	int nid = pxm_to_node(target->memory_pxm);
>>  
>> @@ -882,7 +883,8 @@ static void hmat_register_target(struct memory_target *target)
>>  	 * Devices may belong to either an offline or online
>>  	 * node, so unconditionally add them.
>>  	 */
>> -	hmat_register_target_devices(target);
>> +	if (register_devices)
>> +		hmat_register_target_devices(target);
> 
> Why a new flag to pass around and not something like:
> 
> diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
> index 5a36d57289b4..9f9f09480765 100644
> --- a/drivers/acpi/numa/hmat.c
> +++ b/drivers/acpi/numa/hmat.c
> @@ -867,6 +867,9 @@ static void hmat_register_target_devices(struct memory_target *target)
>         if (!IS_ENABLED(CONFIG_DEV_DAX_HMEM))
>                 return;
>  
> +       if (target->registered)
> +               return;
> +

So this still triggers the lockdep warning. I don't think it's smart enough to know that it gets around the issue. My changes with a new flag does not trigger the lockdep.

DJ

>         for (res = target->memregions.child; res; res = res->sibling) {
>                 int target_nid = pxm_to_node(target->memory_pxm);
>  
> 
> ...?