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

[<dan.j.williams@intel.com>]

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

...?