Re: [PATCH] x86/MCE/AMD: Decrement threshold_bank refcount when removing threshold blocks

[Borislav Petkov <bp@alien8.de>]

On Wed, Jun 15, 2022 at 01:51:47PM +0000, Yazen Ghannam wrote:
> Yes, I believe that's true based on code inspection. But I'm not aware of any
> reported issues in this area before the commit listed above. So I decided to
> switch the Fixes tag from what I had before (shown below). I can switch it
> back if you think that's best.
> 
> Fixes: 019f34fccfd5 ("x86, MCE, AMD: Move shared bank to node descriptor")

Yeah, that is probably the culprit. I finally got to this and am able to
repro on my F10h box.

Here's what I think the fix should be, Greg, please check this
for no-nos, especially for doing a kobject_put() on the parent in
remove_shared_bank_kobjects(). But that is basically the reverse
operation of the kobject_add() I'm doing when sharing the bank, more to
that below.

I wonder why we see this now - maybe the kobject reference counting got
changed since then...

Anyway, thoughts?

---
From: Borislav Petkov <bp@suse.de>

x86/MCE/AMD: Correctly drop shared bank references

Old AMD machines have a shared MCA bank 4 which reports northbridge
error types. That bank has a bunch of controls which are exposed this
way in sysfs on CPU0:

  /sys/devices/system/machinecheck/machinecheck0/northbridge/
  ├── dram
  │   ├── error_count
  │   ├── interrupt_enable
  │   └── threshold_limit
  ├── ht_links
  │   ├── error_count
  │   ├── interrupt_enable
  │   └── threshold_limit
  └── l3_cache
      ├── error_count
      ├── interrupt_enable
      └── threshold_limit

In order to expose the exact same controls - the bank is shared
between all CPUs - threshold_create_bank() reuses the bank pointer and
kobject_add()s it to the parent of the other CPUs:

  mce: threshold_create_bank: CPU1, yes, use it, kref: 4, parent_kref: 3, name: northbridge
  mce: threshold_create_bank: CPU1, inc cpus: 2, bank ref: 4
  mce: __threshold_add_blocks: entry, kobj: 0xffff888100adb218, parent: 0xffff888100c10c00 ref: 1, parent_kref: 4, name: dram
  mce: __threshold_add_blocks: misc,  kobj: 0xffff888100adb418, parent: 0xffff888100c10c00,  kref: 1, parent_kref: 6, name: l3_cache
  mce: __threshold_add_blocks: misc,  kobj: 0xffff888100adb318, parent: 0xffff888100c10c00,  kref: 1, parent_kref: 7, name: ht_links
  ...

kobject_add() does a kobject_get() on the parent for each sysfs file it
adds.

Therefore, in order to unwind the same setup work when the CPU goes
offline and the bank *references* only are being removed - the other
CPUs still share it - do a kobject_put() on the parent.

Rename things more properly while at it and add comments.

Signed-off-by: Borislav Petkov <bp@suse.de>

---

diff --git a/arch/x86/kernel/cpu/mce/amd.c b/arch/x86/kernel/cpu/mce/amd.c
index 1c87501e0fa3..b2bdee9e0bae 100644
--- a/arch/x86/kernel/cpu/mce/amd.c
+++ b/arch/x86/kernel/cpu/mce/amd.c
@@ -1241,31 +1241,40 @@ static void threshold_block_release(struct kobject *kobj)
 	kfree(to_block(kobj));
 }
 
+/*
+ * Drop refcounts and delete list heads in order to free the memory.
+ */
 static void deallocate_threshold_blocks(struct threshold_bank *bank)
 {
+	struct list_head *head = &bank->blocks->miscj;
 	struct threshold_block *pos, *tmp;
 
-	list_for_each_entry_safe(pos, tmp, &bank->blocks->miscj, miscj) {
-		list_del(&pos->miscj);
+	list_for_each_entry_safe(pos, tmp, head, miscj) {
 		kobject_put(&pos->kobj);
+		list_del(&pos->miscj);
 	}
 
 	kobject_put(&bank->blocks->kobj);
 }
 
-static void __threshold_remove_blocks(struct threshold_bank *b)
+/*
+ * Only put the parent kobject of each block. The inverse of  kobject_add()
+ * above in threshold_create_bank().
+ */
+static void remove_shared_bank_kobjects(struct threshold_bank *bank)
 {
-	struct threshold_block *pos = NULL;
-	struct threshold_block *tmp = NULL;
+	struct list_head *head = &bank->blocks->miscj;
+	struct threshold_block *pos, *tmp;
 
-	kobject_del(b->kobj);
+	list_for_each_entry_safe(pos, tmp, head, miscj)
+		kobject_put(pos->kobj.parent);
 
-	list_for_each_entry_safe(pos, tmp, &b->blocks->miscj, miscj)
-		kobject_del(&pos->kobj);
+	kobject_put(bank->kobj);
 }
 
 static void threshold_remove_bank(struct threshold_bank *bank)
 {
+	int cpu = smp_processor_id();
 	struct amd_northbridge *nb;
 
 	if (!bank->blocks)
@@ -1275,14 +1284,14 @@ static void threshold_remove_bank(struct threshold_bank *bank)
 		goto out_dealloc;
 
 	if (!refcount_dec_and_test(&bank->cpus)) {
-		__threshold_remove_blocks(bank);
+		remove_shared_bank_kobjects(bank);
 		return;
 	} else {
 		/*
 		 * The last CPU on this node using the shared bank is going
 		 * away, remove that bank now.
 		 */
-		nb = node_to_amd_nb(topology_die_id(smp_processor_id()));
+		nb = node_to_amd_nb(topology_die_id(cpu));
 		nb->bank4 = NULL;
 	}
 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette