Re: [RESEND PATCH v4 2/2] acpi/hmat: Fix lockdep warning for hmem_register_resource()

dan.j.williams
2025-11-08T01:06:06Z

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.
>
> Split out the target update section of hmat_register_target() so that
> hmat_callback() only envokes that section instead of attempt to register

s/envokes/invokes/

> hmem devices that it does not need to.
>
> Fixes: cf8741ac57ed ("ACPI: NUMA: HMAT: Register "soft reserved" memory as an "hmem" device")
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
> v4:
> - Fix fixes tag. (Jonathan)
> - Refactor hmat_hotplug_target(). (Jonathan)
> ---
> drivers/acpi/numa/hmat.c | 47 ++++++++++++++++++++++------------------
> 1 file changed, 26 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
> index 1dc73d20d989..d10cbe93c3a7 100644
> --- a/drivers/acpi/numa/hmat.c
> +++ b/drivers/acpi/numa/hmat.c
> @@ -874,10 +874,33 @@ 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)

Ah, this makes sense, but is quite a bit of churn with the new
indentation and new function name which is a slightly odd fit since
initial setup is not "hotplug". Is the following equivalent / easier to
follow?

Either way,

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/acpi/numa/hmat.c b/drivers/acpi/numa/hmat.c
index 5a36d57289b4..a1be8cf70dc4 100644
--- a/drivers/acpi/numa/hmat.c
+++ b/drivers/acpi/numa/hmat.c
@@ -878,22 +878,16 @@ static void hmat_register_target(struct memory_target *target)
{
int nid = pxm_to_node(target->memory_pxm);

- /*
- * Devices may belong to either an offline or online
- * node, so unconditionally add them.
- */
- hmat_register_target_devices(target);
-
/*
* Register generic port perf numbers. The nid may not be
* initialized and is still NUMA_NO_NODE.
*/
- mutex_lock(&target_lock);
+ guard(mutex)(&target_lock);
if (*(u16 *)target->gen_port_device_handle) {
hmat_update_generic_target(target);
target->registered = true;
+ return;
}
- mutex_unlock(&target_lock);

/*
* Skip offline nodes. This can happen when memory
@@ -905,7 +899,6 @@ static void hmat_register_target(struct memory_target *target)
if (nid == NUMA_NO_NODE || !node_online(nid))
return;

- mutex_lock(&target_lock);
if (!target->registered) {
hmat_register_target_initiators(target);
hmat_register_target_cache(target);
@@ -913,15 +906,16 @@ static void hmat_register_target(struct memory_target *target)
hmat_register_target_perf(target, ACCESS_COORDINATE_CPU);
target->registered = true;
}
- mutex_unlock(&target_lock);
}

static void hmat_register_targets(void)
{
struct memory_target *target;

- list_for_each_entry(target, &targets, node)
+ list_for_each_entry(target, &targets, node) {
+ hmat_register_target_devices(target);
hmat_register_target(target);
+ }
}

static int hmat_callback(struct notifier_block *self,