Re: [RESEND PATCH v4 2/2] 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.
> 
> 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,