Hi Dave,
On 11/5/2025 3:48 PM, 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
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 <[email protected]>
---
Thanks for this fix.
This resolves a deadlock I was hitting while working on the Soft
Reserved series.
Before this change, the HMEM deferred work path could hold
hmem_resource_lock while eventually triggering memory onlining where
udev holds mem_hotplug_lock.
Tested-by: Smita Koralahalli <[email protected]>
Reviewed-by: Smita Koralahalli <[email protected]>
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)
{
int nid = pxm_to_node(target->memory_pxm);
+ /*
+ * Skip offline nodes. This can happen when memory
+ * marked EFI_MEMORY_SP, "specific purpose", is applied
+ * to all the memory in a proximity domain leading to
+ * the node being marked offline / unplugged, or if
+ * memory-only "hotplug" node is offline.
+ */
+ if (nid == NUMA_NO_NODE || !node_online(nid))
+ return;
+
+ guard(mutex)(&target_lock);
+ if (target->registered)
+ return;
+
+ hmat_register_target_initiators(target);
+ hmat_register_target_cache(target);
+ hmat_register_target_perf(target, ACCESS_COORDINATE_LOCAL);
+ hmat_register_target_perf(target, ACCESS_COORDINATE_CPU);
+ target->registered = true;
+}
+
+static void hmat_register_target(struct memory_target *target)
+{
/*
* Devices may belong to either an offline or online
* node, so unconditionally add them.
@@ -896,25 +919,7 @@ static void hmat_register_target(struct memory_target
*target)
}
}
- /*
- * Skip offline nodes. This can happen when memory
- * marked EFI_MEMORY_SP, "specific purpose", is applied
- * to all the memory in a proximity domain leading to
- * the node being marked offline / unplugged, or if
- * memory-only "hotplug" node is offline.
- */
- 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);
- hmat_register_target_perf(target, ACCESS_COORDINATE_LOCAL);
- hmat_register_target_perf(target, ACCESS_COORDINATE_CPU);
- target->registered = true;
- }
- mutex_unlock(&target_lock);
+ hmat_hotplug_target(target);
}
static void hmat_register_targets(void)
@@ -940,7 +945,7 @@ static int hmat_callback(struct notifier_block *self,
if (!target)
return NOTIFY_OK;
- hmat_register_target(target);
+ hmat_hotplug_target(target);
return NOTIFY_OK;
}
