Re: [PATCH v3 2/2] acpi/hmat: Fix lockdep warning for hmem_register_resource()

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Fri, 17 Oct 2025 14:21:05 -0700
Dave Jiang <dave.jiang@intel.com> 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.
> 
> Split out the target update section of hmat_register_target() so that
> hmat_callback() only envokes that section instead of attempt to register
> hmem devices that it does not need to.
> 
> Fixes: cf8741ac57ed ("ACPI: NUMA: HMAT: Register "soft reserved" memory as a
> n "hmem" device")
Fix up whatever caused that line to wrap!

> notmuch/

?

> Signed-off-by: Dave Jiang <dave.jiang@intel.com>

The guard change seems to be unrelated to rest of the patch.
Probably shouldn't be here.

> 
> ---
> v3:
> - Refactor to split out target device setup vs target update (Dan)
> ---
>  drivers/acpi/numa/hmat.c | 48 ++++++++++++++++++++++------------------
>  1 file changed, 26 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
> index 1dc73d20d989..ddbdd32e79a8 100644
> --- a/drivers/acpi/numa/hmat.c
> +++ b/drivers/acpi/numa/hmat.c
> @@ -874,28 +874,10 @@ static void hmat_register_target_devices(struct memory_target *target)
>  	}
>  }
>  
> -static void hmat_register_target(struct memory_target *target)
> +static void hmat_hotplug_target(struct memory_target *target)
>  {

> @@ -906,7 +888,7 @@ static void hmat_register_target(struct memory_target *target)
>  	if (nid == NUMA_NO_NODE || !node_online(nid))
>  		return;
>  
> -	mutex_lock(&target_lock);
> +	guard(mutex)(&target_lock);
Smells unrelated...

If you did want to do this I'd also do
	if (target->registered)
		return;

	hmat_register_target_initiators();
etc.


>  	if (!target->registered) {
>  		hmat_register_target_initiators(target);
>  		hmat_register_target_cache(target);
> @@ -914,7 +896,29 @@ static void hmat_register_target(struct memory_target *target)
>  		hmat_register_target_perf(target, ACCESS_COORDINATE_CPU);
>  		target->registered = true;
>  	}
> -	mutex_unlock(&target_lock);
> +}